First off, it doesn't matter too much where the horns were put together. Find one that you like to play, and you're in business.
But to answer your question - as far as I understand it, Selmer didn't start assembling horns in the USA until after WWII when certain US trade laws were passed. Any pre-WWII BA is therefore French assembled. BA horns were only made from 1936 - 1947, so there is just a two year post WWII window. If this really matters to you, get a serial number chart.
Yes, often the engraving was different on the two horns (French assembled vs US assembled), but I have seen so many different engraving styles on older Selmers that there were obviously numerous exceptions to the "rules" that way. I'm also not sure if the first US assembled horns were engraved in France or the US ... anyone? I know later US assembled horns were engraved in the states, but in 1946 were they?
But again, I've played a bunch of these horns ... French assembled, American assembled, Commonwealth stamped etc .... it doesn't matter, they all play well. All of these horns were MADE in France btw, sometimes that gets confused. US horns were only assembled in the states, and by now, these horns have been overhauled and repadded often enough to negate whatever differnece in original set up there might have been. Find one you can afford, and be happy.