Whoops
Something (probably me when cleaning up the joint after brazing it together in 2015) must have put a nick into the kit bike’s seat tube, because it just cracked all the way around this morning (it had probably been slowly cracking for a while, but the extent of the crack was hidden in the BB filth) and broke the tube in half.
This is most likely the end of the kit bike; I can cut apart and remove the bits of the seat tube, but that’s a lot of fussy torchwork to get that tube out of the BB without popping the downtube and chainstays out of their lugs. New tubeset time, I suppose; perhaps a set of light True Temper tubing and reuse the current set of hellenic stays?