Brand new Acme differentiating magneto coupler used on the Rumely 20-40 G and some of the 16-30;H Rumelys and others. The OAL is 1.900 inches the large bore is .750 with a 5/32 key way and the mag side is the Large SAE taper with a 1/8” key way suitable to the Bosch DU -2 magnetos .