Nice OM conversion!. Love those grips.
About the biggest bore & highest pressure round you can stuff into an OM 357 Blackhawk frame.

Looks like a 357Maximum cylinder might have been used in the conversion. That gets you extra length at both ends of the cylinder, Back end for recessed rims. Up front for extra cartridge OAL (better bullet selection available to the user).
The bbl length inside the frame window is shorter to allow for the longer cylinder.

The rebated recoil shields give a nice touch.
I like the added side frame rebates as added onto the Colt custom SAA's also. They are copied from early lightweight, short bbl'd SAA guns done by Croft and a few other custom gunsmiths of the 20's and 30's.
Rugers don't lend themselves to the rebates done to the forward side edges of the frame as the Colts do. Ruger places their Ser# there on the RH side. Rebating the frame there will remove it,,a big Don'tDoIt!

But I have seen some Rugers done that way and then the mechanic just restamped the # either there or on the bottom of the frame. Take your chances I guess.
Bowen would never do side rebates on Rugers for that reason (ser#).