Pressure plate - Pre-Verto types.
Apart from very first one that should be avoided at all costs (cast in part no.2A3509), there is an astonishing five other types generally available. For high-tech ones see 'Flywheels & Pressure plates - technical advances'.
22A598 - Number cast in. Standard-grade cast iron
22G270 - Number cast in. High-grade cast SG iron as fitted to Coopers and Cooper Ss Quinton Hazel - Triangular in shape, but only standard-grade cast iron.
C-AHT230 - Mini Spares high-grade SG cast iron, but cast pre-lightened. Farndon Engineering - Machined from steel billet.
The first two can be lightened, see accompanying diagram. Limit removal of metal on the rear face to 0.120" to be safe, 0.150" if you're feeling brave/lucky.