Correct.
In 1973,,, the New Models came out, with a different design in the trigger return spring. In OM's, there is a hole in the frame, that allows a spring & plunger to sit in, which is used to return the trigger forward. With the New Models using a totally different design, they milled a slot, drilled a hole for the legs of the spring & all.
Now, that slot in the NM g/f's can be filled with a "block" that has a hole in the end for the OM styled spring & plunger, and it's held in place using the cross pin hole in the frame. These adapters are not hard to make. I can see if I can get pictures up later.
But, as for making an OM g/f work in a NM,,,, it will require a lot more work. AND,,, most of us PREFER to just sell an OM g/f & buy a NM,,, as the OM's are worth more.
As for when the XRN-3 g/f's were made & used,,, I can't recall off the top of my head. Just know that it's a NM g/f. The actual shape of it where the grips are concerned is the same as the XR3-RED. So, when buying grips,,, you can use grips for both frames. If my faded memory is right,,, the first g/f's in the NM's were marked; NXR3-RED. After that,,, I can't recall.