Michael (Vernal) kindly sent me a photo of the 'double' breech seal and I am pretty certain the mystery is solved as it looks identical to the seal for an Hw77/97. I have held an HW seal up against 2 x BS108 seals and they are remarkably similar and I can believe they occupy the same space.
So... out of interest, has anyone fitted an HW seal in a TX or even used 2 x BS108 rings in an HW77? Maybe the HW would need a shim?

And, if you are going to remove the end plug... 27mm is the size. The end is not very deep so make sure the spanner / socket fits well. I needed to heat the end with a gas torch to soften the sealant ... not very hot but hotter than can be hand held. I heated for maybe 10 or 15 seconds playing the heat around the edge of the plug and it undid easily. When I did it, the fitted O ring came out unscathed by the heat.

Cheers, Phil