They are very easy to disarm. Unscrew the fuse, pull the pin and let the fuse burn out and screw it back in. Perfectly safe. We used to do it all the time in Vietnam to screw with the newbies
One of my old bosses (an Army sniper in VN) said that troops used to unscrew grenades and use the explosive to heat their rations in the field.
Yes, but yours had not been sitting in someone's attic for half a century. It might have been so corroded as to make the fuse very difficult to remove. Plus the stability of the fuse would be in question after that long.
Old ammo is one thing, old grenades, with the pin half out, are something quite different. Still the police needed to have their fun too, blowing the thing up.