I have a 261 Canadian Chevy engine from a big truck with a block casting number 3837012. The casting date is not in the same location as the USA blocks. It is behind the intake and exhaust manifolds. (must remove to see) All of the Canadian blocks I have (235 and 261) have a capital letter M located inside a square on the passenger side of the block between the distributor and the draft tube. This indicates it is manufactured by McKinnon industries Canada.

1954 Chevy 1300 engine 235 (1959)
1955 (first series) Chevy 3100 engine 261 (1957)
1927 (other brand) Model T- engine 327 (1968)