Part of the thing trying to get across here Mick is that because of the design and thickness of the hawk seal, is that the volume of the hole in the washer should also be taken into consideration? (the volume of the seal must be about 1/3 of the volume of the transfer port?) Ie that the hole should be considered a part of the transfer port in just the same way as if you fitted a sleeve to the transfer port that extended as far as the front edge of the seal and then for example fitted an O ring for a seal instead of the Webley job. I can see you trying that soon to save on Webley seals except I bet you turn your own.
On something like a meteor, with a very short port and a seal on the barrel it would be a different case.
I wonder why the small Webley and Diana rifles had a washer on the cylinder side? I think on the Diana 23 it was a very shallow recess as well?