[Arran]

Ron;
  I don't know if this will work on electrolytic filter can #3 or not but I see that it has a stamped steel nut, sort of like a tinnerman nut. If it refuses to come off you should try prying the ears, on the inside of the nut, away from the threads by inserting a screwdriver or awl between the ears to lever away the tension, you only need to go about 1/2 way around to do it. It may not work but it's worth trying if it makes it easier to work on the can. With regard to the Philco can I would be inclined to save it for the next Philco that needs it, and find a generic substitute like an old Aerovox, or just leave the labeling as it is, it's part of the radio's history.
Regards
Arran