This is what I could salvage/find from the the old PGPOG Faq

1. General Information about PGP

This document is intended to answer common questions related to the care, use, modification, and repair of the Sheridan/Crosmann PGP paint marking pistol. Much of this information also is applicable to the other brass tube construction pump action paint markers produced by the Sheridan/Crosmann company.

PGP Operating Diagram

I am writing this FAQ for the people have questions about the PGP.

I copied a lot of information from the PGP FAQ from PGP Owners Group and the “Old vs. New” article from Stock Class Paintball. The underlined parts are what I copied from there.

1. General Information about PGP
2. How to take the PGP Apart
3. Aftermarket Parts (Upgrades)
4. Troubleshooting
5. How To ….
6. Misc & Credit

First thing first, what is “PGP”?
The PGP is a pump-action paintball pistol which has been in production since the early 80s. It’s predecessor was the Sheridan PG, which required the user to twist and pull the bolt to the rear to recock and chamber a ball. The magazine was also located on the side, which required tilting the gun ‘back and to the left’ to load. A sort of ‘missing link’ model has recently come to light, which had the top tube like the PGP (which eliminated tilting the gun left) but also had the twist/pull cocking action. 

What does “PGP” stand for?
No one really seems to know for sure. “Paint Gun Pump” is my personal guess.

How old is my PGP? 
One key is the pump handle. The PGP has gone through several changes…here they are from oldest to newest: Steel pump (knurled), Aluminum Pump (ribbed), Delrin Pump (ribbed), current plastic pump. The valve type (see below) was changed at some point in between the Delrin pump and the new plastic pump era. 

Serial numbers mean nothing on Sheridans as chaises were built in batches and then may have sat for years and years before being assembled into guns. Guns them would sit in warehouses for years too before being sold to shops. Some shops would then have guns for years and years before selling.

Not to mention the company has been bought up three time since the production of that gun. As buy out go the information is lost and destroyed.

What type of valve is in my PGP? 

If your PGP looks like this in the lower tube, you have the newer cartridge valve.

If your PGP valve looks like the one above, with the squarish depression around the valve stem, then you need a Sheridan valve tool to further disassemble your gun.
Sheridan valve tool – Available at Mad Custom and Palmer’s Pursuit Shop.