Thanks to all for all the suggestions so far.
To answer your questions, this is a custom design in a custom mold, lathe bored by David Mos, a respected mold maker. For unmentionable reasons it can not be replaced. I have used the bullets in competition off and on for the last 20 years and it has produced tens of thousands of bullets. From time to time I have adjusted the protrusion of the alignment pins while checking progress with a mike but this doesn't seem to completely correct the mismatch. The sizing die, of course makes the driving bands round. I have also tried sizing the nose with an NOE nose sizing die. This makes the nose round but doesn't improve groups and sometimes seems to make them worse.
After careful cleaning, I can't see light between the blocks or feel any motion.
I will working on it tomorrow and let you know if any progress.