You can try Matt, as mentioned by Phil, or perhaps Chris Cloke can give some pointers as to how he made them when he was running Sheerline.
I think they’re moulded from some form of RTV rubber, perhaps silicone or urethane rubber, most likely into machined plastic mould, or perhaps they moulded them in-situ on the endcap.
Another method would be to cut some out of stock rubber sheet, but that could be quite expensive, as most of the rubber will be cut away to form the ring.
I’m not sure if there was any advantage over using o-rings, perhaps they wanted a flat seal with plenty of compliance. The snag is that unlike an o-ring, they’re not a stock component.