Okay this may not be something you want to read but I just bought a master cylinder for my 90 in Mar 06. AC Delco p/n on box is 18M399. There is another # on the box, 18030641, I guess that's a good GM # as it comes back as a master cyl on two GM dealer sites' search engines.
The master cyl is a PBR/ made in Australia in an AC Delco box. I bought the rebuilding kit from RockAuto.com and it is an AC Delco kit in an AC Delco box listed as a wagner or raybestos kit(?) on the site, I forgot which.....I haven't yet done the work but it looks like everything the FSM lists is in the kit....I may send it off to a rebuilder locally to have mine sleeved first & have them put it together along with a DRM bias spring.
I can't explain why your #'s and the #'s on the master I just bought (bought it from the last detail in Atlanta) don't match. Did you get those #'s from the parts CD that whiteracing.com sells? If so those are original 90's #'s and alot have been changed over time by GM.
Tom