Look it over real well...the little "proof" marks will lead us to the actual maker.
Will find a bunch of marks on that barrel somewhere...often hidden by the stock....some will be provisional proofs, definitive BP proof, usually a letter code for a date, and a maker's mark of some sort (either a design or a series of letters, often intertwined).
Once we know who made it, willb e able to trace down the right nipple thread. It proably came with a musket cap nipple (as almost alll muskets do), but replacement nipples using a #11 cap are available. Would stick with musket caps on a gun like this.