No-one has mentioned adjusting the dead volume of the pump.

I got one these pistols in the 70s after covetting my mate's one - I only had a crappy break-barrel at the time. I simply used the piston position adjustment to change the dead volume at the end of the pump stroke. The trigger was never very nice and it was certainly less nice after the adjustment. My point is, that indicates that an "adjustment' had indeed been made.