According to this website and also my own experience with disassembling and documenting these computers.

The very easy way to tell the difference between the 4 types of tower models is to specifically look near the power button area on the front and the LED/buttons in the front. Pay attention to it carefully.

All tower models come with a 386 CPU.

Generation 1
MODEL 1, MODEL 2, S1, S1

Parts are not as interchangeable. Requires a SCSI card in order to use hard drives which are external only. Power Supply card different than other 3 generation tower models. CPU has a socket so it can be upgraded.

Generation 2
“F” Family of parts
CPU has a socket so it can be upgraded.

Generation 3
“0F” Family of Parts

Generation 4
CX Family of parts. Has FAST mode for software that supports it.