Hatsan Hercules Bully Breech Seal Dimensions

Any chance someone knows what the breech seal dimensions are for a .357 Hatsan Hercules Bully? The part number is 2609 from Hatsan's exploded diagram. For some reason I can't logically comprehend, they don't list any dimensions on the manufacturer site.

I've already made an inquiry with Hatsan but don't expect a very prompt response... surely someone on here has dealt with this before, help me out please!

Thanks in advance!!