It is a half moon key. I guess that the tension of the belt would also prevent it from coming off. I will tap it back on and replace the belt. I hope to start it up this week for the first time in 46 yrs. I have followed the manual for timing, new plugs, wires, points, coil, etc. and excited to see if it will run. Thanks for the input. Will post as to how it goes.
Jim

Do You own a car built by Durant? D-40