The general information about publishing and Nirvana's publishing companies over the next four paragraphs have been compiled from various sources, but each specific source will not be linked
A music publisher manages the ownership, promotion and administration of songs. They promote and sell licenses of words and music (typically a song or a production music track) to record labels, radio, television, films, commercials, and multimedia. Here, a song is not the physical recording of the song, but the composition. A particular sound recording of a song is owned the by record label.  One can say that the "true copyright" is registered with the publisher as written in the copyright text on an album or single. In effect, a released song then has two copyrights, one in the musical and lyrical composition, and one in the actual released sound recording.
It is also a music publisher's task to register works of songwriters and composers with all appropriate performing rights organizations, which will collect the money on behalf of the publisher for a fee. The remaining money will be split between the publisher, record label, and the writer(s). 
When you write a song, you automatically own the copyright. You may decide to publish it yourself, in which case you will have to start your own publishing company to manage the copyright. To avoid all the complex hassles with publishing, many turn administration over to a major publisher. In an administration deal, the major publisher doesn't own any equity in the copyrights. They just collect a percentage of the money that comes in and pay the rest out to the writer and the writer's publishing company. In a co-publishing deal, the major publisher and the writer's publishing company each own 50% of the copyright. In both these cases, the major publisher does all the work. Your publishing company just holds (a part of) the copyright to take care of your interests. Alternatively, you may sell your entire copyright catalog to a major publisher. 
Nirvana founded the publishing company The End Of Music, but had Virgin Songs Inc, BMI to control and administer their publishing. Virgin Songs Inc, BMI later turned into EMI Virgin Music Ltd, and finally EMI Music Publishing. A large stake of Nirvana's publishing rights was sold by Courtney Love to a newly-established publishing company, Primary Wave Music Publishing, in 2007. The stake is bigger than Dave Grohl and Krist Novoselic's combined. The remainder is still administered by EMI Music Publishing .
Several "publishing promo" submission discs have been issued to promote the songs, mostly for inclusion in film soundtracks. These discs are usually on recordable media. As these discs are not proper promotional discs in the traditional sense, they usually don't have catalog numbers, and they usually don't credit the owners of the particular sound recordings featured on the discs. They are just tools in the publisher's work, and as such this is not required.
Before 2006, only a very few publishing promo samplers were known, the most curious being the 2002 Nirvana For: Daredevil submission CD-R with six tracks; Blew, Paper Cuts, Dive, Stain, Downer, and Scentless Apprentice. None of the tracks were included in the film. In 2006, the Film Soundtrack Division of EMI Music Publishing issued CD-R publishing promos of all the band's records, including the box set and Sliver The Best Of The Box. One such publishing promo can be seen in images 1-2. This type of proactive approach was relatively new to music publishing, long a passive business in which rights holders mostly waited for potential licensees to come to them .
Shortly after purchasing their 25% Nirvana stake, and also publishing rights to many other artists, Primary Wave issued a publishing promo sampler CD featuring Smells Like Teen Spirit and Come As You are, and a limited to 500 five-disc wooden heart-shaped box set titled The Songs Of Kurt Cobain, featuring album and cover versions of Nirvana songs. Both these items were factory pressed CDs. More information and pictures can be found at nirvana-discography.com.
1. 2006 Incesticide EMI submission CD-R, disc
2. 2006 Incesticide EMI submission CD-R, insert
3. 2007 Primary Wave Volume One, front
4. 2007 Primary Wave Volume One, back
5. 2007 The Songs Of Kurt Cobain box set (missing outer cardboard box)