All information from Mr. Labo's website

While the tower models are more iconic due to the vertical drive the desktops without hesitation have more power than their tower predecessors.

All desktops generally have 4 expansion slots in the back with one for a video card and 3 for general purpose cards such as a MIDI card. They also have CPU upgrade sockets. Later models require them to function as they don't have a DX2 or above CPU soldered to the motherboard. All desktops are also capable of FAST mode. Some parts are interchangeable with one another as far as power supplies and CD rom drives. Generally it should be visually obvious which models belong in the same family of parts.

To add more RAM to a desktop model remove the top cover. On the left side near the power supply harness should be some RAM slots

Pay attention at times to the disc drive cover as well. It usually is an obvious way to denote the model if you can't see the dataplate in the back.

HG - 386SX CPU

HR - 486SX CPU

ME - 486SX CPU

MA has a 486SX

MX has a 486DX. Generally parts can be swapped between MA and MX

FRESH / MF / Fresh･TV has 486SX. Pay attention to disc drive cover.

Fresh E / ES / ET / FS / FT vary with CPU.

This desktop has only 3 expansion slots in the rear. Note that due to the height difference it's very peculiar in terms of compatible CPU upgrade cards.