Some context for the adventure here. The question comes in the last paragraph.
I bought a long model Onoto 6234 from Spain recently, expecting it would need some service work. The pen looked to be in very fine shape externally, so that was promising.
The plunger was extremely difficult to push down. Before taking it further apart, I removed the section to add some silicone grease to the barrel, as an experiment. Now the plunger moved very easily, but with no suitable 'pop' of vacuum breaking.
Removing the rod showed that the "cup washer" was made from two bits of what appears to be bicycle inner tube (it curves floppily on one radius) hand cut with a knife into a random polygon roughly approximating a circle, with no cone washer for support. OK, I can replace that. A lack of ink in the barrel or section suggested the earlier repairer had found their effort was not entirely successful.
I noted also that the plunger cone pin had rust around it! Pushing it out, I found that my predecessor had resorted to a bit of metal for the pin, and not a rust-proof bit, for this pin immersed in dyed water.
Also, the rod was bent, presumably from trying to force down the ungreased piston. Well, it is ebonite with a wire core. I am trying to ease its bend at the moment. It may need replacement.
Next, I turned to the cork seal at the top of the barrel. Hmm, no seal screw. That does not help. So, to the question:
Does anyone know the threading used for the seal screw on top of an Onoto barrel? Given they used 5BA for the rod, I am conjecturing it will be 0BA or 1BA (but normal RH thread). If it is, I may be able to make another.