In the US, Ford used Flathead engines in all their cars and trucks up until 1954, when 339 cu.in. Y-block V-8s and Mileage Maker straight 6s came out.
272 and 292 Y's came out in 1955. No Flatheads.
312 came out in 1956, then Ford supercharged it in '57.
In 1958, Ford cars came with Y-blocks but Thunderbird used the new FE and MEL engines exclusively.
So, especially during the '50's, it wasn't hard to tell the age of any engine. Today, we look back on casting numbers because there were so many revisions and improvements.
The only engines that were used in '55 thru '57 were Mileage Maker 6s, 292's and possibly some 272's. 312 didn't come around until 1956. I'm going to exclude the Lincoln Y-blocks because they came out in '52 and were totally different engines.