I agree that the grips don't fit right, and I'm inclined also to think
that they are not original to the gun. I can't tell about that dark line
on the right grip panel - it looks more like a grain line, but maybe its
cracked there.

The gun doesn't have a lot of monetary value - maybe $150 t0 $200.
As a family heirloom, its your call as to whether spending $300 or
more to get it refinished , is worth it. With a good nickel refinish, it
might help the value somewhat, being as how your cost basis is
probably zero.

The thing about these M&P's is that there is a lot of them - like
hundreds of thousands, so unless one is in 95% or better condition,
they are not worth much, monetarily. And a refinished example of a
very common gun is never worth much, anyway.

Mike Priwer