Saw this for sale locally, guy said he was told it was the correct frame for his Ruger (don't know what he has), but it doesn't seem to fit. He wants $25, brand new as far as I know, but could be used.
From my list of grip frame Founder’s Marks CAST ON GRIP FRAMES here's what it is:
New model Large alloy (#XR3-RED OM size), the most common grip frame; used on Single Sixes and large frame Blackhawks. W/o lock: Early = NXR3-RED, Late = XRN-3RED.