Japanning is the black stuff under the shellac. Note that I didn't recommend bathing the machine in rubbing alcohol - I said some spill over won't hurt the japanning. By this point in time, any shellac is dried out drek anyway.
Japanning on metal is (in Singers case) a baked on finish usually made with asphaltum, turpentine, lamp black and some other stuff I can't remember. I think I saw an old Singer video where they were baking the coating on in multiple coats. Japanning was used extensively on industrial equipment exactly because of it's durability.
Frankly, I think the rubbing alcohol (45 to 65% water anyway) would evaporate too quickly to do any damage, even in larger quantities. 190 proof Everclear, on the other hand might be just the ticket to remove that dried out old shellac ... I think some experimentation is on the horizon.