CMA Steam/water Valve Seat Seal
15 x 4 x 4mm. EPDM rubber, 80 Shore Hardness. For Astoria and Wega, this gasket is held in place with a tiny fixing nut. They are usually quite brittle when they wear out, and must be pried (and likely chipped) out of the gasket holder with a sharp tool. They are then installed in any position/fashion.
WooHoo no more struggling to get the steam valve to turn off completely. Old washer was as hard as a rock. The new one seals so easily.