Arnold, I agree that it is a triangle and an original foundry mark.
Original Ford Model A cylinder heads manufactured during production had either a straight line, 45 degree line, or a triangle on the top of the head between the two upper water pump mounting studs. This foundry mark was used for QA (Quality Assurance) to identify the source of discrepant parts.
Cylinder heads manufactured later and sold at Ford dealers as replacement parts were marked differently.