Led Zeppelin - Hey Hey What Can I Do / Immigrant Song single

I searched the forums, and all I found was this thread:
http://www.stevehoffman.tv/forums/showthread.php?t=19939

I have a copy of this single. It's a reissue on the gold/black Atlantic "Oldies Series" label. Both tracks are in stereo. However, I was surprised to find some familiar codes in the deadwax:

Side One: Immigrant Song
ST-A-19953-6 AT GP 18-70 (Specialty Logo)

Side Two: Hey Hey What Can I Do?
ST-A-20450-4 AT GP (Specialty Logo)

Obviously the work of George Piros.

I found it odd that side one has a completely different code (19953) from side two (20450). I don't know much about 45 RPM issues, so maybe there's a reasonable explanation.

Am I correct in assuming that side two used the 4th stamper, while side one used the 6th stamper?

What codes are on the original pressing?

When was the "Oldies Series" 45s made?

What is the best quality version on vinyl?

What should I be looking for?

I don't have the box set to compare with my record, but it doesn't sound GREAT. It's ok, but nothing to write home about.

 

Also, did HHWCID ever sound better on 45 than CD?

 

Possibly on the Japanese 45, but it has been so many years since I have actually heard it. Maybe someone who knows where their copy is (unlike myself) can actually confirm.

 

Am I taking this Led Zeppelin matrix-thing too far?

Probably.

 

Nope, that's why we're all here.

 

Not stamper; lacquer. Big difference.

But as for why one number is lower than the other, it may have had something to do with when Atlantic assigned master numbers to each song. I recommend (if you visit, say, the Performing Arts Library at Lincoln Center) a book by Michel Ruppli called The Atlantic Discography. That should tell you when the respective master numbers were affixed to which song. Obviously they were assigned at different times, after they got the master tapes from England. . . .