These M, N and P molds could have been used by Courtauld’s in Wrexham (licenced LEGO production) or by Borg-Warner in Grangemouth (production of raw ABS for LEGO). We do not yet know enough about these bricks to determine their origin with any certainty, but all seem to originate in the United Kingdom.
Small amounts of bricks made in molds N and P have been found together in matching colors. We did not find enough bricks yet to determine what the entire mold looked like.
It is possible that a single test mold was made out of reused elements from two old production molds.
[Original post and comments on Flickr.]