Back to the OP's question.
As CraigC mentioned, the aftermarket industry has produced Ruger compatible grip frames for years.
Here's a brass XR3-RED on a OM Single-Six.
This pair of shooters sport the same grip frame as pictured above...
Here's a S.G. marked brass Super Blackhawk style that's configured in the New Model style (easily adapted to OM, though.)
Back in the late '60's/early '70's Ruger sold brass Super Blackhawk grip frames, too.
A company in California made brass Ruger Grip Frames back in the late '80's.
There
was an outfit in the Carolina's, I think, making Brass Bird's Head grip frames and then, of course, there's Ron Power's style....
flatgate