I found these at their site, they refer to it as a valve seat.

Ultra-Gate 38 Hi-Temp Accessories: Part number

Garrett 32 adapter FG-WG38-AGRT
KKK (small) adapter FG-WG38-AKKK
Spring 3psi outer FG-WG38-SP03
Spring 5psi outer FG-WG38-SP05
Valve Locking Tool FG-WG38-VLT
Double Flange Kit (stainless) FG-WG38-DSF
Pressure Line adapter FG-ADA-DASH4

Ultra-Gate 38 Spares: Part number

Silicone Nomex Diaphragm FG-WG38-DIA
Belpa Mica Manifold Gasket FG-WG38-GSKT
Double Flange Kit (mild steel) FG-WG38-DWF
Spring 7psi inner FG-WG38-SP07IN
Spring 7psi outer FG-WG38-SP07
Spring 10psi outer FG-WG38-SP10
Valve Seat FG-WG38-SEAT