Jer, after looking through some GM Canadian service bulletins from the early 1950's, it turns out Buick used a different numbering system on engines built in Canada.
Attached is a page explaining engine numbering for 1953 started at B119001. This confirms your engine B124557 came from a 1953 car with synchromesh transmission. It can't be later than '53, as that was the last year for the straight eight.
Other typical bulletins show running changes made at 1951 engine B105140, 1952 engine B115170, and 1955 engine B154443. So it looks like Buick was building about 10,000 engines a year in Canada back then.