Here is a look at an early PMI 3 that was likely sold in 1990. Serial on the left side is 505. The PMI 3 featured a longer bolt, shorter hammer, and a longer cocking slot on the left hand side. Early models also featured match anodized feedblocks, trigger guards and side plates. This model has an after market sight added onto the feed block and and muzzle break.
When the Sheridan released the VM 68 they added ambidextrous cocking, a shorter bolt and longer hammer and the side plates were mostly plastic which cracked apart easily.
Find VM-68 and PMI 3 parts for sale at http://www.baccipaintball.com/parts/semi/vm-68.html