That big Flathead engine is one of the U-series but I can't tell the exact model and year from the photos. According to Bruce Palmer's restoration book the medium-compression Model U and the high-compression Model UL were both 74 cubic inch and were both built for 1937--1948. The medium-compression Model UH and the high-compression Model ULH were both 80 cubic inch but built for 1937-1941 only.
The engine has aluminium heads and it also appears to have a ribless gear/cam cover but the presence of those things is inconclusive regarding the engine's exact model and year because of parts interchangeability.
There was also a 1939--1940 Model UA which was a low-compression 74 cubic inch model built for the US Army. And a 1937--1939 (or 1937--1940) Model UMG which was a limited-production magneto-generator equipped bike built for the NYPD. Eric
