What is confusing to me,, is the following; " two 1960's 3 screw revolvers with the XR3-Red grips frames."
These would have to be post 1963. And they will be Old Models.
Now,, the New Models and especially in recent years,, the grip fitment has been spotty. But all the OM XR3-RED grip frames I've seen or own have never been "undersized by close to an eighth inch on the front strap and the back strap." And in the 1960's the medallions will have a recessed eagle, black in color. The later XR3-RED grips will have a raised eagle that's silver.
It strikes me as very odd.
Can you post pictures of the grip frame markings? Can you post pictures of the grips on the guns?
I'm NOT saying you don't have an issue,, or anything like that,, I'm just confused.
And some of the posts above,, pertain to the NM design,, and yes,, you can have fitment issues with a NM.