The Schwinn GT510 was supposedly an improvement on the insane Schwinn GT500.
The Allen key hanger bolt has been replaced by a riveted on hanger, with a dodgy hidden plastic insert playing a key structural role between the hanger and the body of the derailleur - probably not good. You get a funky black drop out bolt and adjustable cable outer stop. Weirdest of all you seem to get one of Huret’s toothless ball bearing mounted tension pulleys - very hip in 1960.
What anyone in France or Chicago can have been thinking about speccing this gear in 1978 is beyond comprehension.