My best guess would be 46 to early 50s. I don't think those drop outs with brace tabs started until 45 or at least appeared in the catalogs in 46. Sprockets would probably define the latest year it could be, but I'm not sure when they stopped being used. Sounds like these Schwinn brazed frame tandems are near impossible to date by serial number, but there should be one under the crank.
Also, I guess it helps if I hit the Post Reply button. Ha
I agree with the back hub not looking correct. Could be S4 rims, correct for that era.