Smg68 (like A5 only longer)
Sl68 (slip fit)
Proam (commonly called A5)
Model 98 (2 slight variations with the most recent working in all of them)
I'm not super familiar with the slipfits bit I *think* they were all interchangeable.
Smg60 to smg68 changed due to barrel size. I'm guessing the sl68 was slipfit because that was popular in the early 90s (quick access to clean the breech). Proam continued the smg68 thread but didn't need to be as long as it wasn't the stripperclip setup. 98 was for the casting reasons andrew listed. I believe the revision in 98 was for longevity reasons.
I believe the Inflicted managed to fit one of the Mac solenoids Deathwish is selling into his older Ego. If you search around, you can probably find his pic of it. It would then be a fairly simple fix for about $20.