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M_V
I have two oldish Thorns. Love em. Massive steerer stacks, I’m proud to report.
I have a Thorn eXp of some vintage that has a lot of similarities to the one you’re describing. Fillet brazed. ‘Sealed’ top and down tubes, closed bottle bosses, tapered main tubes. For derailleur. Chunky ends with socketed tops that may well be ritchey under the powder, never looked hard. My pump peg is down the NDS seat stay, the peg mounted on a little curved bracket spanning the seat tube to seat stay. Mine has an X shaped bridge instead of single seatstay bridge.
As far as I’ve discovered the tubes are likely to be what Thorn were calling their custom ‘conical 725’.
I believe mine’s made by Kevin Sayles when he worked at Thorn SJS. Let’s hope he sealed mine properly. The only way to find out would be to saw the tubes or drill holes. Did the one that came to you crack somewhere?
I did have a chat with Dave Whittle a while back who I think mentioned there were also conical 853 tubes at one point. But one would imagine 725 would have been used on the heavyweight expedition type frames.