I have seen an engine with the looks of a 1953-55 first series engine, like this one, but cast much later. That engine had the passenger side pan rail bulged out like the 1955 second series-63 engines(1953-55 first would be straight down both sides). I would check the pan rails to confirm.
I just re-looked at the pictures of this engine and see a raised circle above the starter. I have seen this marking on another engine which was reported to be a 261(maybe Canadian).
