When Schwinn’s archaic Chicago factory (and equally archaic industrial relations) couldn’t keep up with booming demand, it decided to import bikes, made to its specifications, from Japan. To do this Schwinn needed a ‘Schwinn Approved’ derailleur made in Japan - and turned to Shimano to provide the solution. Schwinn’s Chicago factory used the ‘Schwinn Approved’ Huret Allvit - so Schwinn demanded something similar from Shimano, including all the Allvit’s nastier features:
Shimano duly obliged with this strange creation, part Huret Allvit, part Shimano Pecker and part Shimano Lark.
It weighed a ton, but it quickly became a favourite with Schwinn Cyclery mechanics because it worked so much better than the Allvit.
One other distinctive feature is the beautifully machined steel pulley wheels (which are also found on some Shimano Cranes - a derailleur that was also supplied in quantity to Schwinn).