Apparently the Potters have some form or center board down locking mechanism. I can understand why this would be important so why do the M's not have that capability?
There isn't much weight in the M-boat CB. The ballast is in the fixed keel and the CB is really only there as a hydrofoil. For comparison, the Compac 16, with a similar configuration, only added the CB recently. I've found that the CB makes a big difference to the M-15 when beating.
The Potter accounts sound like they felt that the board should be locked down at all times
I haven't sailed a P-15, but with dinghies of similar hull shape and size I've pulled the board up on a run. On the other hand, I don't mind capsizing a dinghy (much). Giles Morris Arlington VA Vancouver 25 #002 "Dolphin" Montgomery 15 #264 "Umiaq" Sundry kayaks