Brass 3/4 inch BSP BS1212/1 part 1 float valve (also known as a ball-cock, inlet valve, fill valve or ball valve).
This size of ballcock valve is more usually found in an industrial environment. You are ** VERY VERY UNLIKELY ** to have this in your house (unless you live in a really huge house). Most houses use a 1/2 inch valve.
Threaded ball mounting end has 5/16 inch thread - use a 6 inch float.
** BSP thread sizes are measured a special way - have a look at our information page for more details **