"Mexican" test pressings are retail records where the retail labels have been removed, and replaced with adhesive labels, plain white or appearing like pressing plant labels, to mislead buyers into believing they are purchasing actual test pressings. To my knowledge, this was first attempted around 2005 by a Mexican individual who claimed to be an important figure in the EMI Capitol label. He made such "test pressings" of the following regular US 7" singles:
Text like "PRUEBA TEST PRESSING EMI CAPITOL MEXICO 1991" was etched directly into the vinyl run-out grooves together with the proper matrix. The counterfeitor's explanation was that at the time Geffen was looking for someone to distribute Nirvana in Mexico, and they contacted three labels; Warner, RCA (later BMG), and EMI Capitol. Geffen sent them some advance 7" promos or test pressings pressed without labels, so that the three labels could come up with a marketing and promotion plan, and Geffen would decide which company to go with. The test pressing labels where then attached internally by EMI Capitol. As if this was not unbelievable enough, the counterfeitor's eBay history showed that he just before had purchased US retail records of the mentioned singles. He also attempted to sell the records without any labels at all, in case somebody wanted the pure test pressings.
1. Mexican counterfeit test pressings
These records were pressed by GZ Vinyl as determined from the matrix codes, the same Czech pressing plant which manufactured the official Nevermind records. However, no retail records have ever surfaced. The records are:
2. Czech samples
Nevermind does not contain Nirvana music, but two songs from two unknown releases . The two 12" records have all the songs on side A. Nevermind first surfaced before 1996 , while Scratch It Out/ Bikini Twilight was sold on eBay in January 2002 .
At least two Nevermind records have been found, one with white vinyl between the two pictures, and one with pink or purple vinyl. One was discovered in Australia. The original seller claimed to have gotten it from Geffen Australia, and that it was a visual test pressing to show what the pressing plant could do . The story is not as far fetched as one would immediately think. At the time, there were no domestic pressing plants which manufactured picture discs in Australia , and at least in more recent times I have seen GZ Vinyl pressings for sale outside of the Czech Republic, probably due to cheaper prices.
However, what makes the story more questionable is that it was also said that it was for a once planned 2x7" release of the album . That would mean three songs on each side, which would lead to horrible sound quality, if it is even possible. It is more likely, if it even is a sample, that the pressing plant used whatever pictures and stampers they had available at the moment to make some samples showing what they could do, which could have been sent to any record label.
It is also curious to note that the pictures are cropped from the official Nevermind 12" picture disc, as shown in image 4. The center of the 12" picture can be seen, as they shifted the center.
3. Official Nevermind 12" picture disc, showing how the 7" pictures were cropped
It is harder to see how this would apply to the two 12" records. They were both discovered in Japan, and only one of each has been found. It is possible that they are samples to inspect the sound quality, but the stickers seem unnecessary for this purpose. I am not sure if it has been confirmed that they even contain the music claimed by the stickers. It seems unlikely that they would make brand new stampers for a general sample, when they could just have used existing stampers from any other release they were pressing.
None of these appear to be regular bootleg releases. Only one or two have been found of each release, and this is obviously not profitable. However, they are currently listed as unofficial. Even if they are samples, they could have been sent to any record label.
This appears to be a test pressing of an old counterfeit. It comes with generic test pressing labels with handwritten catalog number, "SP-23". The matrix codes are, according to the eBay auction the picture below is taken from, "Why Don't You Trade Those Guitars For Shovels" SP23A L31540 and SP23 L31540X (may be slightly inaccurate).
4. Old counterfeit Love Buzz test pressing
A Love Buzz white label 7" was also produced similarly to the "Mexican" singles described further above, though not by the same person. The retail labels were removed from an original retail single, and white adhesive labels were attached, or they were attached directly on top of the retail labels. The singe was in poor condition and probably missing the sleeve, which could be the reason why the owner decided it was worth an attempt.
5. Counterfeit Love Buzz test pressing
Picture disc with blank pictures and black vinyl in the middle layer. It may be a test pressing of an actual counterfeit pressing, or a counterfeit pressing made to look like test pressings. The matrix codes are GEF 2445P A2 and GEF 2445P B2.
6. Unofficial blank picture disc
The counterfeit Simply Vinyl white label records have the matrix codes SVLP 038 .A and SVLP 038 .B. They can also be identified by the vinyl color. Copies have been found with either lots of green or red mixed in with the black vinyl. They came in picture sleeves.
7. Unofficial white label, red vinyl
This white vinyl pressing first appeared in 2002. All copies came with blank, white labels, and in picture sleeves. At least one copy came with handwritten information on both labels, and came only in a white inner sleeve. Matrix codes are DMX79745 SIDE ONE/TWO.
8. Unofficial white label on white vinyl
Regular UK single with white sticker labels placed over the regular retail labels. Various notes were handwritten on the label and white sleeve. Matrix codes are DGCS 7 A-1U-1-1 Stu. and DGCS 7 B-1U-1-1.
9. Sides A and B sticker labels
An unofficial marbled brown vinyl was released around 2006. White label records were also pressed. Both variations came in picture sleeves. "Muster" is handwritten on one of the white labels. It suggests that the records were made in Germany, as it is German for "sample". The records appear yellow when put in front of a light source. They have no matrix codes.
10. Unofficial white label on brown vinyl
The album, released by Anja Records in May 2000 in the USA (TAO 360), was not authorized by Dylan Carlson, Earth's frontman. The songs were bootlegged from DATs stolen from Carlson by a former bandmate, who refused to hand over the tapes. Around seven white label, black vinyl test pressings were made. The retail pressing was on clear vinyl, 360 handnumbered copies. Matrix codes are L-10403M-A/B M.M.M.J.R.  The test pressings came in white cardboard sleeves with a sheet of paper with photocopied artwork. Instead of being numbered, "TP" was written.
The release featured the tracks Divine And Bright (mislabeled as Divine Bright Extraction) and A Bureaucratic Desire For Revenge, both with Kurt Cobain performing the vocal tracks. 
11. Label and photocopied artwork
White label records for the unofficial second pressing also exist, supposedly 25 were made. They were pressed using the same plates, meaning the matrix codes are identical. The vinyl may have some lighter streaks compared to the solid black first pressing, but otherwise impossible to tell apart. According to myth, a record store owner in Los Angeles tracked down the pressing plant, pressing plates, and the original sleeves while Josh Hunt, owner of Autofact, was in jail. He convinced the pressing plant to make a new batch of records which he sold on eBay, to distributors, and to other record stores. It should be noted that this story cannot be substantiated. 
A third pressing was planned, whether that would be official or not I don't know. It was cancelled, but at least two yellow-green white label copies have surfaced, both from the Autofact owner.  The same plates, or at least stampers manufactured from the original mothers were used, so test pressings shouldn't have been necessary. Most likely the white label pressings were just made to make an extra buck by selling what appeared to be test pressings.
12. Second pressing test pressing, side A
13. Second pressing test pressing, side B