What you're talking about is the New XR3 steel grip frame first brought out with the New Vaquero in 2005.
I am a grip maker myself and as far as I know the following guns use that smaller XR3 grip frame.
New Vaquero
New Vaquero (Montado)
.357 & .44 mag Anniversary flattops
.44 special flattop - both the Lipsey models and the now regular Ruger cataloged gun
So if it is a New Flattop, it will have that New XR3 grip frame on it. You know of course that flat backed grips for these frames need a recess in the back to clear the locks, right?
The regular Blackhawks and Single Sixes have the internal locks but they still use the XR3-RED sized grip frames on them.
One sure fire way to tell which frame your customer has is to get him to measure it's width. The New XR3 grip frame is .437" in width whereas the rest of Ruger's plow handle grips are still .500" in width.
What's in demand from me mostly is the New XR3 and the XR3-RED. With the New XR3 currently being the most called for.