It's a PMI, but not a Sheridan. In the way back Sheridan made markers for Pursuit Marketing Inc. PMI started selling blowbacks later on without Sheridan. I guess these go in the Miscellaneous Marker section.
Yours looks like a GT eforce, with a CO2 expansion chamber. They work well as they are, but don't get a whole lot for upgrades. You can render them relatively fast (say 12+ bps) with HPA and some work, but they are hard on paint. One good break can bind the bolt up pretty tight sometimes.
You see a lot of the mechanical versions as field rentals. They are simple, inexpensive and mostly reliable. Boss Paintball may still make a straight aluminum trigger for that frame, which was a lot nicer than the stock tongue.
That may be a PMI regulator, not an expansion chamber. I forgot for a moment that they had some rather large regs at one point.
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