I never saw a measurement for the seat tube, inner and outer which could help define the tubing and provide more insight, it's one of the last things possible. Rather than suffer through measuring the inner, may want to measure the outer, then measure the tube thickness.

If you already posted it, I must've missed in the flurry of pics. Oh, and as far as the numbers stamped into the bike ... unless they follow a recognizable pattern, chances are they're not going to help. I don't see anything following a pattern or any marks or indications of origin.

I think the head badge rivet spacing will help unlock the mystery, but I feel like the sweet spot of BF's C&V knowledge is really post-bike-boom (late 60s onward, of which this bike appears a bit earlier. If you haven't yet, you may also try to post this on The CABE. Some members posting in this thread also participate over there, but I feel like there's a bit more resources familiar with older non-US items.