RB 41 for the fluted and RB 41W for the roll marked.
All blued Bisley Blackhawks started with 7.5" barrels.
And while it's true they are fairly rare, esp. the fluted model, they do come up more often than one would think. Last year, I saw 2 fluted 41 Bisleys and at least 5 roll marked ones come up on GunBroker and I probably missed some. Also when searching GunBroker, you can search "Ruger Bisley" or "Ruger 41 Bisley", but don't forget to just search through the ones identified simply as "41 Blackhawk". That's how the frame is marked and some uninitiated folks don't know it's a Bisley when they advertise them. Good luck in your search--keep looking, one will pop up soon.