My mistake!~ I assumed it had a pressure tank inline behind the handle, like many other models! Does the pump (that I don't have) just screw into the threaded port at the back of the handle?
Currently the spear will launch, but without much force. It should work fine with a pumped up charge. Do you know the recommended PSI?
Yes, you attach a pump into the valve at the very back of the gun. But the challenge is finding out what kind of thread Technisub used in that valve. Perhaps they adhered to one of the two major sizes used by most other brands. If that's the case you can pick up either a Cressi pump or a Mares (Seacsub uses that too as well as some older Omer guns).
But I guess there at least two more standards. Some years back, another forum member had gotten his hands on an old Scuba Pro gun and since the molds of that gun was later put into service to make a new line of Salvimar guns, it was a very reasonable suggestion that a Salvi pump would fit his gun, but for some reason it didn't. So, for screw-in pumps, I guess there are Mares, Cressi, Salvi and possibly a Scuba Pro standard. Maybe even more.
Do you have a way to measure the thread diameter and pitch of the female thread of that valve? If you still have the plastic plug - and if that one is a male "match" for the thread perhaps you can let us know its size and we can try to make an informed guess for you. Also, let us know if you can see an o-ring sitting below the thread in the valve? Reason being, some brands have the o-ring on the pump, others have it in the valve and it might help is hone in on an answer.
For now, I can tell you the following about the pump threads:
Mares: M14x1, o-ring on the pump
Cressi: M18x1.5, o-ring in the valve? (not sure at all)
Salvi: M14x1.25, o-ring in the valve
If none of these match, you might have to have an adapter made. If you end up going that way, I suggest you pair it with a Mares pump as they are the easiest to source.
@popgun pete What do you think? Looking at the drawings, it seems to me that the inlet valve might be pinned in place on the barrel as opposed to being screwed into the barrel? If that is the case, I guess swapping the whole inlet valve is out of the question.