As I recall, the collar you are describing tightens around the distributor shaft housing to keep it in time once properly adjusted. There should also be a flat plate which takes a 1/4 x 20 bolt into a tapped hole in the block. This assembly also has the vacume advance mechanism. That's the way it's set up on my 216 and I beleive it was on a 49 235 I had. I know it was also on my 52 Chevy coupe. On the 216, this plate has degree markings. You can move the timing a bit either way by loostening the hold down bolt and twisting the distributor. This was provided to adjust for octane variations.