This would have been one of the frames made by Don Mainland in his machine shop in Racine WI. I talked to Marc Muller a couple years ago about how to tell Mainlands frames from others. He said they're marked with a P for his company name "Pioneer". I've had the frames marked P4 so this was likely a Mainland built all chrome frame. Apparently Schwinn had Mainland build the ones slated for all chrome because of his brazing skills. I'm guessing the extra numbers were put on by Don Mainland and this was an all chrome frame from the factory. Oh, it should also be stamped P4 on the bb shell but probably covered by paint now.
Ben Cole
Springfield IL