So, default wallpapers in early Windows 95 builds.
The first Preliminary Development Kit had an "Under Construction" wallpaper. The second one had the same wallpaper but tiled. Beta 1 had a different "Under Construction" wallpaper.
Of course, after PDK1 leaked far and wide, MS implemented some serial protection in PDK2 up to beta 2 (different from the one in the RTM; however the RTM's setup has remnants of it). Given that the leakers were involved with the warez scene, however, the skilled reversers that the builds were passed to easily found the backdoor that had been put in so that those on MS' internal network didn't have to enter the serial, and just patched a few bytes in the setup to abuse that.
The interesting part of this serial was that it was in two parts. One part was the "beta site ID" and half of the "password"; if this was valid, but the second half of the password wasn't, setup would appear to continue... until the point it would copy files, upon which it errored out with a message "General error 57, please contact your beta administrator".
This misdirection was discussed at the time, with some people believing the error at face value. This continued with the foundation of communities to preserve and discuss such builds several years onwards.
The last half of the password was the hex form of the first 16 bits of an MD4 hash (this code was written in around autumn 1993!) of the beta site ID, the first half of the password, and... a string inside the setup that was used as the titlebar text for the error message, which would be something like "Microsoft Chicago Preliminary Development Kit 2, November 1993".
This part of the serial algorithm was finally reverse engineered, and a key generator made....in 2014.
Afterwards, some early Internet Explorer 4.0 builds were discovered. They used the exact same serial algorithm as the early Windows 95 builds. And had the exact same "MS internal" backdoor.
(Um, I think I may have just ruined one of Raymond Chen's future blog posts.)