Assuming that no brass has fallen off, it looks like there's a big gap between the end of the down tube and the head tube, and the brazing hasn't penetrated and filled the gap. It looks like bad workmanship to me, the joint seems to be held together purely by the lug, there's no direct contact between the DT and HT. As already mentioned, the brazing doesn't seem to have penetrated the bottom part of the lug either.
I agree, as a complete layman in these issues I would have expected to see more brass in there. Maybe the DT has been hanging off the top of the lug for 23 years rather than being supported evenly all over by a better braze.
It could also be just "one of those things".
I'm over it now, I'm glad to have walked away from it unharmed and glad that I found a probable reason behind the major 'Shimmy' I had on the Bryan Chapman 600.
The more I think about the high-speed shimmy on BCM the more I don't want to think about it, having felt what happened when the downtube snapped at walking speed on Sunday.
So, thanks for all the kind words, yes it was a lovely bike (if I say so myself) but I still have all my teeth, I'm not lying in hospital encased in plaster, it's just metal when all said and done and I just received confirmation that my Cyclescheme voucher has been approved.
Next post in this thread will hopefully be the Condor Fratello, a bike I ordered as a direct modern equivalent to the Touristique and, gorgeous in its own right.
Funny how things turn out.