This is a machined from bar Aluminum Prop Driver uses a Tapered Collet to lock it on the crankshaft.
The first K&B and Veco .61's used a cast driver that was held in place with two flats on the crankshaft. You may use this on the older engines but you must also buy the collet PN 19-6101

1.050" diameter and .330" total thick.