The original '70s spec. fluid would have been DOT3, and the Lockheed master cylinder reservoir caps normally say: "USE ONLY DOT3 FLUID FROM A SEALED CONTAINER"
These early Triumph* M/Cyl. caps appear to have had the '3' of DOT3 added as a stamp mark and later a printed sticker was applied containing the number information in print.
*The different style of Lockheed plastic cap (as fitted to the Norton Commando 850 Mk3) have the lettering moulded into them.
However...DOT3 fluid has been superseded by DOT4 fluid which has a higher specification, so the answer would be any normal DOT4 universal brake and clutch fluid.
Lockheed do not recommend the use of silicone (DOT5) fluid, but I've heard of riders that have used it, but the two types of fluid should not be mixed.