What you seem to have there is a franken PG...
It is a PG body and bolt with possibly a home made grip frame.
The PG (Persuit Game) predates the PGP (Persuit Game Pump) It was actioned by pulling the bolt back with your hand.
As for the leak. It sounds like the cupseal is blown. You will need a Sheridan valve tool to remove the valve. You can buy them from Palmers or from www.tituspb.com
There are detailed instructions on how to remove the non cartridge valve here on PBN and on MCB.
Once you have the valve removed you should replace both the cupseal, as well as the Orings, and the face seal as they are probably ALL old and rotton.
Replace the valve and you will then have one very fun little pistol!