1956 Chevy 3100 235 straight six:

I bagged what I thought was the hold down for the distributor about a year ago. When setting up the distributor for correct timing today I discovered that the hold down was not correct, but the bolt, washer and lock washer that I had in the bag appear to be correct IF what holds the distributor in place is the advance/retard collar on the shaft of the distributor. I started to attach a large O-ring in the top groove of the shaft, but then decided not a good idea and stuck with just the gasket that seats nicely underneath the advance/retard collar.

So, the question is, is there actually a hold down for the distributor (and no place for it to hold down on the distributor) or am I correct in figuring the advance/retard collar serves as the hold down?