My light bulb went on (about 40 watts occasionally ;-) ) when looking over a No.1 and pondering the loose cap problem. FWB recommends the RWS #1075 cap which is #10 size and initially needs to be pinched to stay put. Either FWB, or Billinghurst back around 1850, fit the pistol with a straight neck nipple! What's with that, there is no way to utilize a cap's crimping without a taper? Not a good design for an underhammer with nipple pointing to the ground. The quickie fix would be to add a thin smear of high-temp solder to the base of the nipple. This would eventually burn off and/or scuff off and need to be reapplied, but it might be the answer. Otherwise, you will need to determine the thread size and go shopping for tapered alternatives of the correct length.