You've got yourself a nice old P-gun!
Basicly it is a long barrel, Direct fed, Constant Air powered version of the PGP.
Actually, there is quite a bit that can be done to it. Alot of mods were done to those guns, and I am still very happy to play with my old PGPs and varients.
Did you check the date on the bottle? 5 years from the date and you shoudl be fine. Also you might want to try a smaller 7-12 oz tank, because the gun is fairly heavy, and the extra weight of a big tank is fairly uncomfortable (not saying anything about your sex, I prefer the 7oz tanks myself)
Look up:
www.docsmachine.com
He takes a while, but can do everything to that gun, even make it a Semi
http://www.palmer-pursuit.com/
They are the ones who did some of the first mods to old brass pumpers, and are still some of the best.
So, how did it shoot?
Josh