Oil Seal
I'm not sure of the exact name for the black ring, but I think it serves as an oil seal - keeping crankcase oil from escaping via the base of the distributor.
The split pin is what keeps the oil seal in place. You will need a very small guage drift to drive the pin out. If you can think of a nail set that isn't tapered. I fabricated a drift out of a nail set, myself. But, that's the only way I know of to remove the black ring.
Also, if you look at the pin, you will notice that it has a smaller diameter on one in than on the other. Drive it out from the small diameter end.
I am not totally sure that you need to remove the seal unless you also want to remove the distributor shaft. For purposes of replacing the pick up, you probably do not have to remove the seal. But, if you want to inspect the distributor's weights, you have to remove the shaft.
By the way, why are you replacing the pick up coil?
Don