Welcome to the Forum! You inherited a nice .38 M&P revolver, and they are very popular here on the Forum. It definitely shows its age, and as you know it could use a little TLC. Before you do any cleaning, remove the walnut stocks (grips)...those you can clean with some Murphy's Oil Soap Wood Cleaner and a soft toothbrush. Mechanically it looks to be in VG condition, and it should be a fun shooter. The innards could be a little sticky and dirty with old grease, oil, and gunk so you might want to spray some CLP or automotive brake cleaner inside to remove those residues. You could remove the right side plate to look inside, but I suggest you watch some YouTube videos on how to do it correctly without damaging the parts. There were literally millions of M&P revolvers manufactured by S&W, so your grandfather's is neither rare or particularly valuable. However, it is a fine 100-year+ old family heirloom and should be around for a long time to come. Enjoy!