Interesting about the Hollis and i think you are right they probably have been recut to take readily available nipples. You would not necessarily see where a good craftsman has been! I can't believe Hollis would have used UNF, or SAE (Society of American Engineers) threads as would have been at the time (same thread form).
All my English original guns are 1/4" BSF taking No. 11 caps, except a large fowling piece which has a military style nipple taking 4 winged musket caps. The thread generally used on these is 5/16" BSW. Hope this helps.
