I thought the Si-Tech rings had the drysuit seal on the outside of the ring and to be honest, the rings are particularly thick so I doubt it makes that much difference. However, the rings I have seen might not have been Si-Tech, I just took the owners word for it
Regarding a stockist, PM Digger as I am sure he has Si-Techs.
haven't held them in my hands, but looks like the seal goes outside a small ring, but only stretched to the i.d. of the big spanner ring