Excellent read ... thank you.
Now I somehow doubt if this is relevant and I may have mentioned it before but I am interested in your comment about a 100 series carbine. Some years ago I bought what was described as an Air Arms Chameleon .177 rifle at auction. It was covered in camo tape, action, stock, the lot so a thorough examination was not possible. On getting it home the tape was stripped off to reveal an obvious 100 series action type; a chrono test showed it to be way ott. A strip showed a Cortina suspension type hammer spring and a sort of home made valve head in that the old head had clearly been loose and araldited back on. The action had been 'carbined' by the simple method of cutting the end of the cylinder off, mostly the removable end cap area and welding the air filler valve threaded part back on. The filler valve was thus still removeable but as there was no end plug there was no internal seal. The trigger unit was the match trigger unit. I corrected the power issue but after very few shots, the valve head broke away ... a new replacement was not available but I did try making one. This had limited success as the rifle would only hold air for maybe a month. The cylinder seals were U rings, not standard O rings. I found a supply of these from a local firm.
Interestingly, the user manual (I used to have an SM100) gave instructions on how to adjust the power ... before the AT scene arrived on the block.
I eventually got bored of experimenting with trying to make a new valve head (and failing) so sold the rifle as a project.
Cheers, Phil