Interesting thread. I agree with Flatgate, I do believe some .44 Flattops as well as some .357 Flattops were fitted with XR3-RED grip frames toward the end of Flattop production. I have probably seen and handled 8 - 10 in all my years. As we all know, Bill Ruger did not throw anything away and when engineering/parts changes were made I believe the new and old were mixed and overlapped. The small serial numbered Supers with a mixture of Micro sight, steel ERH, Alloy sight, alloy ERH during the transition is a perfect example. Never a certain exact time this part was ended and that was part began....just like serial numbers are not done in sequence. While there is normally no way to validate a particular grip frame from the factory I have seen a few and bought some that I really felt was "right". Two main factors that help validate in my mind that they certainly can be correct is the later ship date and the uniform graying color of the anodizing on the grip frame....then just the look of the gun in general. At least every one that I felt was "right" met those standards.
I bought a boxed .44 FT with XR3-RED from an elderly man at a small gun show who said he bought it new. I asked him if he had ever done anything to it, changed anything, etc., nope, that was just the way it came....he never mentioned anything about the GF. I believe him, said he bought it in 1963, he had no reason to lie and sold me the gun for only about $400. Turns out that it lettered as being shipped June 1963 and of course it had that uniform graying GF. I also own #42658, the .357 FT with XR3-RED discussed here and it is the only one that I have ever seen that I felt was correct as soon as I picked it up. It meets the characteristics I mentioned above, shipped November 1962 and has the uniform gray anodizing, so I do feel that it is original and correct....can I prove it from the factory, no, but I like it, treasure it and really don't care much beyond that.
It is sad that folks will change things on guns but that is just a fact of life. As a collector who wants 100% original I hate it but its their gun and they can do what they want with it. Like Mike said for a comfort thing or maybe to make a buck. Its up to me to do my homework and make a decision as to a gun's validity.
All of this is just my opinion after many years of studying, chasing and handling a whole bunch of these old Rugers.....and I have had a whole lot more fun doing it!!
FT44/aka Bill