I bet that company would make the throats whatever you request. I was a little intrigued as well. Mr. Bob, it would make a great addition to your old model 44 Special.
I suspect he makes them for the .429 barrel that Ruger uses on their 44 spec/mag revolvers (and their 44-40s in the past).
Ruger under throated the cylinders on their 44-40s. I would just send those cylinders back to Ruger (or Hamilton Bowen on occasion) and they came back opened up to the correct size and accuracy was just dandy!