This brick was produced from a mold that The LEGO Group in Denmark provided to the Borg-Warner plant in Grangemouth, Scotland, where they produced ABS (plastic) pellets used by British LEGO Ltd. to manufacture bricks for retail. The mold at the B-W plant was used to test the plastic formulation and colors before sending a batch to British LEGO. The marbled bricks were likely made by two rogue B-W employees shortly before the test mold was to return to Denmark.
This is a thick-walled brick with lengthwise center rib, PIP (plastic injection point) on short side, and tubes with an outer bevel.
Inscriptions on bottom side: