Hi there, I bought this parker duofold cap recently, but the problem is it seems to be larger than a duofold junior cap but slightly smaller than a duofold senior cap. It won't thread onto a duofold jr. and is very tight against the 5 or so duofold senior barrels I've tried. If it helps, my duofold jr's cap diameter measures 11.8mm, while my duofold senior is about 13.4mm, and this mystery cap is 12.8mm.
Left to right - duofold sr., mystery cap, duofold jr.
Left to right - 3 duofold sr., mystery cap