I don't know if LEMO makes the HD 800 connectors or if Sennheiser makes it themselves but it's custom. I searched for the model number on LEMOs website and none of the connectors fit the one on the HD 800. The FGG.0B.302 is really close but it's not exactly the same. Every compatible connector is HD 800-specific and it costs a ton compared to standard 3.5 mm or miniXLR. AliExpress has some knock-offs but the fit is quite bad. I guess they did it so people will have to buy their cable, which is also quite expensive BTW.