The stop at the end of the spiral groove is because the B engine had pressurized cam bearings and main bearings. Pressurized oil would still find its way beyond the end of the oil channel in the cam bearing. At the back end of the cam, the stop was intended to diminish the tendency of the rear cam bearing to leak out the flywheel cover. At the front end of the B oil galley, there is a hole that dumps oil on the timing gears and feeds the rod dipper tray. On an A cam, since they are gravity feed, the groove extended beyond the bearing to a notch at the base of the block cam bore at the thrust and onto the spinning timing gear which threw it all over tarnation and the teeth as well. The dipper tray was replenished by oil return from the return pipe and what ever came down the timing gears from the top.
You can interchange the camshafts with no harmful effect, except that an A cam in a B engine is a step backwards in performance. It would not hurt anything if you used an A cam core with a B grind in an A engine and cut the groove to help oil the cam timing gears, but with the oil mist all over in an A engine, I doubt if its necessary.
BTW, An A cam reground to B specs will not actually perform as well as a good B cam or a new B grind camshaft. You cannot duplicate the original grind on a reground cam shaft, you can only get close to it. You will either have correct lift and diminished duration or diminished lift and increased duration. If the cam was heavily re ground, you will find you have to extend the adjustable lifters to near the top of their range. If you used the small foot adjustable lifters on a B grind, you would decrease duration even more.
