Re: Remington 51 .380
I would replace the grips if I could get good repros. Replace both, though, as they look funny when they don't match.
Just FWIW, I have agonized a bit about repro grips. The purist collectors may (will) disagree, but I think good repros are better than cracked or broken originals. When dealing with the situation where only one is broken/missing, and the repros are good, I have often replaced only that one to keep the gun as original as possible. But where the repro is not a true match, I will replace both, as I think the match is more important than having one original grip.