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I wonder if like most Theoben things they where slightly thinner breech blocks on earlier models, the one I have is 17.5ish wide with a 16mm barrel girth, the Supersport barrel is 15.5mm wide, so the Theoben block needs to lose 2mm to fit in the breech forks.
Everything seems to line up with a BSA barrel block, even the BSA wedge lock fits in the Theoben block, the barrel pivot pin hole is smaller, so would need opening out to fit the BSA pivot bolt.