Just found a bolt/nut from a BSA Mercury-S which might work, either with a collared nut or by countersinking the breech jaws a bit. It is a loose fit in a HW35 breech block, so the HW bolts must be 7mm not 6.35 mm, and would need the breech block to be drilled....
The BSA bolt is too short so I've ordered a shouldered bolt off the evil bae...