Looking at the 1940 passenger car Vehicle Info kit, it says that the engine serial numbers start at 2697268 for Flint plant engines (Tonawanda plant engines have a B suffix). So if a factory serial number it makes sense that "I" would be a 1 (one) instead, putting it as the 517,406th engine off the line at Flint for 1940. Not sure if there were that many engines produced that year though. From the 1939 Passenger Car Vehicle Info kit, Flint engine serial numbers started at 1915447 and Tonawanda engines at B-10503. So half a million passenger car engines produced at Flint in 1940 sounds reasonable.
Another clue to the year might be the transmission serial number (assuming it's original to the engine.) For 1940, Saginaw plant numbers started at 0000001 and Muncie plant numbers starting at 3000001. In 1939, car transmission serial numbers were prefixed by SG at the Saginaw plant and MY at Muncie.
