Note, as shown in your first closeup pic, that the gun was made from a .20 cal Sheridan pellet gun, probably an EB, rather than being, say, a rebarreled PGP or PMI-1. Also note there's no locking bolt (either spring-loaded balls or "speed demon" type bolt) and no "quick strip" notch.
Making that a pretty early MacMurray Annihilator, that is not only remarkably complete but remarkably original too. Check the hydro date on the tank- while not definitive, it ought to be a good indicator as to it's age.
And, it probably doesn't need to be said, but I'll say it anyway: Do Not Alter Or Modify It! Clean it carefully, yes, but carefully. Remove dirt and dust, but not the finish or "patina". Don't polish anything, don't replace anything, don't lose anything. That sucker is a collector's item.