With my older Chevy trucks ('37, '42, '50 and '51), I have seen discrepancies in casting numbers before. I believe it is because the master parts list purpose is to allow the parts man to order the proper part, or block in this case. So, if a later part number is interchangeable with the original, GM will list the later subsequent number only. A list from the late forties will not necessarily have numbers present on an original '37 block, for example. You would have to have access to a '37 or '38 list to see that number.
In 1937 the 216 was first introduced and the block had a single hole in the front for the water pump. After 1940 the 216 had 2 holes up to the end of the 216 in 1953. So 1937-1940 blocks are the same as far as I know and a later master parts list will only give you the last number available as the other earlier ones are superseded and not available anymore.
Bottom line is not all casting numbers are present in a given list.
Kent
