The existence of the number there tells you it's a Canadian block, the 81 means 1938 introduced 24-stud 221. BUT there were 2 of those...if the end of the crankshaft is pretty much flush with front cover opening, it is a 1938 only engine that uses same crank as 1937-8 21 stud engine, if it sticks out about an inch it is a late 1938-41 engine.