Gentlemen: According to the "Schwinn 1973 Specifications for Paramount Models..." brochure:
"The P-15 Paramount is equipped with the new Schwinn-Approved GT-300 "Le Tour" derailleur with outstanding wide-range shifting capabilities and the Campagnolo "Record" front changer....The P10 has Campagnolo "Nuovo Record" rear and "Record" front derailleurs...regina chain and fw, Bottom bar shifting levers are standard with Schwinn-Approved ratchet-type handlebar shifters available at extra cost. Schwinn Twin-Stick [headset mounted?] controls can be supplied at no extra cost."
In 1972, the equivalent specifications brochure still called for the Campagnolo Gran Trashmo rear on the 15-speed.
Of course, the "as built" might have diverged from the specification. I have a Crane, but not a GT-300, but don't remember any difference except the markings; could be wrong.
thaks
harvey sachs
mcLean VA