FYI the Model 42 is the predecessor of the Model 642. These two guns are "Airweights" with aluminum alloy frames, whereas the frame of a Model 40 is steel.
Also, it should not require an excessively firm grip to actuate the grip safety. The mechanism can get sticky from long-term neglect and cause trigger problems. Dry-fire it a few times while holding it in a normal firing grip, feeling for any hesitation or "hitch" in the trigger. IIRC a sticky grip safety is not necessarily difficult to fix if you know your way around the inside of a J frame, but not everyone does, or wants to.