Thanks for sharing!
The geometry is really interesting. 20" TT and 12.4" CS is one thing, but 73.5 HT? Long time ago when this was available from KGB and Quamen...
I am a little bit surprised by the TT/CS combination because this will bring the seat towards the back. But the way he rides it (slammed, not grabbing it any more) it should not make such a difference for him. Space under the downtube would be similar with a shorte TT and steeper ST (would be interesting to know what it is like here...).
Anyway, he rides so good and must know what suits him well!
Good luck with the frame Benjamin!