From this thread: https://www.boardgamegeek.com/thread/1417822/usa-forces-west...
OK, got it. Mark has a good point.
In 8.42, "USA Forces" is capitalized, meaning it is used in a game-defined sense, with a reference to 8.45 USA Forces. Speaking of USA Forces that "arrive" does distinguish them from units being built by Production.
There is nothing in the rules to indicate that building USA units by Production is treated any differently than for any other Satellite (remembering that they can't leave the USA until the West is at War).
So if the USA becomes a West Satellite in 1936 Diplomacy (3 Influence), USA cadres can be built in 1937 Production and onward by the West (though it is hard to see a reason to prefer them to building British/ French units at that point). Interested to hear the situation where this might be desirable . . .
IMO this makes sense for this game: if the USA is a bonafide West Satellite (military ally) and the West is at War (seemingly the only situation where building USA units would be desirable), there is no reason the USA should have to wait until 1942 to build military units.
PS: There is no way USA units can be built in 1936: the USA cannot be a West Satellite until 1936 Government (Diplomacy), which happens after 1936 Production.
So, yes. You may build up USA blocks early but: 1) USA must be a West Satellite. 2) They can't leave the USA until war is declared.