OEM Windows 95 keys came in the form of XXXXX-OEM-00YYYYY-ZZZZZ. The XXXXX grouping represents when Microsoft issued the key by day-of-year and year, and the operating system would validate any number where the first three numbers range from 001 to 365 (or 366 for year 96), and the last two X digits were 95-99. The YYYYY group must be a multiple to 7, excluding the number 0. The ZZZZZ group is basically noise and anything is permissible in them.
Now you can run a Windows 95 keygen in your head. You're welcome :)
Actually I believe Windows 98, Me, and 2000 have a similar scheme but they obfuscate it with some algorithm to generate the 25-char alphanumeric style that Microsoft continues to use to this day. I've never dug into how it goes, but you can see the 95-style key on System Properties post-install.