Many cast iron radiators have modern fittings that can be adjusted by the radiator keys covered here.
Some new cast iron radiators, catering for those who like the retro look, have standard-sized connections from new. Others have been adapted to the new fittings with reducing bushes. These are cast iron connectors which make it possible to screw modern fittings into old radiators with non-standard internal threads.
A reducing bush has an external (male) thread that fits into the old radiator and an internal (female) thread that will take modern-sized fittings.
There are specialist firms that can provide cast iron fittings and radiator tools, including keys, in original sizes. For instance, you might find a replacement bleed valve which is operated by a hand-turned, serrated wheel.
Cast iron radiators are usually fitted with valves that are in keeping with the style of the radiator, often using standard 1/2 inch valve tails.
If the radiator keys covered do not fit your radiators, you could seek advice from a cast iron radiator specialist, as specialist tools might be needed.