Bisley Single Six hammers have been unavailable for quite a while through the usual sources - Midway, Brownells, etc. Ruger won't sell them separately anymore either. I'd like to find one myself for a similar conversion. Good luck in your search!
I have indeed firelapped several, and the constriction in the .45s seems to be more pronounced than in the .44s I've seen, I assume because of the thinner material at the threads in the .4s. My experiences lead me to believe that rifling wear in the photo is from the firelapping process. I don't...
The eroding leading edges of the lands are from the fire-lapping. The abrasive wears that edge first as the bullet hits that edge hardest as it launches before it starts to rotate. I've seen that done on a couple of stainless .45s; the rifling erodes well before the "choke" is ever lapped out...