(Started with SN 0, ended between 93xxx - 104xxx):
The first generation Universal M1 Carbine mainly used USGI parts, including a USGI bolt locking mechanism. One example in the sub-5xxx SN range had an IBM barrel with flaming bomb emblem.
The transition in production between first generation and second generation is somewhere between 93xxx and 104xxx.
(Started between SN 93xxx - 104xxx, ended between SN 176xxx and 187xxx)
The second generation is where Universal got creative, started redesigning things, added the second recoil spring, and started to machine their own parts. This model eliminated the bolt locking mechanism all together. These had a leaf style flip rear sight, round bolt, magazine catch marked M, and new trigger housing. These could still accept the USGI trigger housing, and some had the M2 style metal upper handguard. The front sight was held on with a setscrew, rather than pinned as the USGI is.
The transition in production between second generation and third generation is somewhere between 176xxx and 187xxx.
(Started between SN 176xxx and 187xxx, went until end of production)
The third generation included cost saving designs that started the company's reputation for poor quality. Changes included a slide lock lever by the rear sight, the trigger housing was made of aluminum, and several safety features were deleted to reduce costs. This resulted in the ability of the rifle to fire out of battery.
The highest serial number documented so far is 486xxx.