It is clear that what you need is the description of US patent 1,924,729 to L. J. Weber (1933). This is I think the beginning of patenting for the US style of ALCLAD. I could not find it online, although many publication refer to this patent.
However find as an attached file patent 2,240,940 assign to Aluminum Company of America dated September 1940 which gives various experimental compositions current in think in 1939.
An important part in this last patent is the text "a small amount of one or more of the grain refining elements titanium, boron, zirconium, molybdenum, tungsten, cobalt, chromium, and vanadium, avoid to a large ex tent the above named disadvantages usually associated with aluminum base alloys containing substantial amounts of zinc".
If one of these elements is present in the sheet of metal you have in possession, it is likely that it can not come from a piece of metal commercially available in 1937
Edgard