WRT the plug -
I initially installed a plug in my 438, but ultimately removed the plug and just left the hole. Although a failure with the plug is highly unlikely (I’ve yet to hear of it, ever), its installation does leave extra parts INSIDE the frame. Should those parts come loose (again, I doubt it would happen), then those parts would be floating freely in the frame.
My feeling is I don’t like the lock, and I wanted all the extra parts gone. The hole is ugly, but it doesn’t harm anything.
Whatever you do, don’t remove the flag and leave the lock barrel. That would result in a lock that could be mistaken as working (it’ll still turn), but not actually lock. This is dangerous as a future owner of the gun might not know that the lock doesn’t work.
Anyway, the OP bought a gun without a lock - good choice!