Your replies are sadly not specific! There is a collar/base for Delco distributors and one for Autolite distributors. The difference is there is a slight difference in height. The Delco item is slightly taller so if you use it on an Autolite distributor, the tabs on the distributor shaft will not engage the slot on the intermediate shaft in the cylinder head. You crank over the engine and it won't start. You take off the distributor cap and find the distributor shaft is not rotating.
My FC170 set up for 2 years because the collar was seized up on the distributor. I turned the distributor upside down and applied penetrating oil at the base. In the mean time I looked for another collar because I figured if I got rough, it would probably break the collar I had. I found another collar at a KF National and asked John Parker about how you could tell the difference and he had no advice.
Fortunately after continually soaking with penetrating oil, I got the collar off without breaking it. In the mean time, I had tried another Autolite distributor with the collar I had bought and as a result, discovered the difference. I didn't have a complete Delco distributor or I could have had the FC running again sooner.