Following on from David's suggestion I contacted the royal armouries museum.
They sent a list of drawings that were possibilities, there were three which sounded good so i bought copies. The one below seems to be the accepted design, its not quite complete as there must be a sleeve or adaptor that fits the square end and acts like a lock.
The design calls for the gun frame to be modified and the bracket bolted onto the frame. I think between us we came to a similar conclusion to what the drawing shows, an interesting question has been answered I think. Hopefully the drawing for the sleeve will turn up in the achieves, knowing British design it'll be one of hundreds of drawings all called "sleeve", no mention of what its for.
looking at the drawing I suspect it was some sort of forging, post machined to fit the gun and mount. There's enough detail and information to allow me to make an adaptor to fit the mount.
Ben
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