I don't think he does, I think you could just use a shoulder bolt and a nut. Bowkett may have used the HW bolt, which is wider and so some machining is needed. The lock-up on the Meteor is OK, the jaws will spread if you over-tighten the stock bolts or if they see heavy use, the trick is to squeeze them back into alignment with a padded vice with the barrel out. The actual jaws are hollow so you can't thread them, you have to use a bolt and nut.
Mercuries can be retro fitted with a Mercury-S bolt and nut but also need some machining.