I wish to replace a door lock set (whatever it is called). The existing one is over 40 years old and I cannot figure out how to remove it. There are no visible screws. The knob comes off by depressing a spring-loaded pin and pulling it off. I cannot see how to remove the rest of the unit.

Can you post a picture. Generally there are screws on the inside (which may be hidden by a flange) that hold in the flange on the outside.

Check bottom of flange. See if there is a slot in it.Put screwdriver in slot and twist. This should pop the flange off. There will be 2 screws holding plate to door.

Remove screws and lockset should come off.

Good luck

Chuck