These are original MGA drain cocks for radiator and engine block. Don't let the color fool you, the parts are made of brass (except the spring). In the disassembled views you can see two slightly different styles, particularly in the way the rotational stop washers work. The one on left (from the engine) has double stops, while the one on right (from the radiator) has a single stop surface. Functionally identical, they have different service part numbers, one being installed by the radiator manufacturer, and the other being installed by the engine assembly plant. The thread is 1/4-inch BSPP (British Standard Pipe Parallel thread). The standard thread designation is G-1/4. This is a Whitworth form parallel fastening thread, 19 threads per inch, and major diameter is 0.518-in (13.157-mm).

3H576 (from the engine) ARH1050 from the radiator
Thanks to Jim Cheatham in Amelia, Virginia, USA for the pictures.

On June 21, 2018, Jim Cheatham wrote:
"Is the original radiator drain cock a restoreable part"?

Yeah, I think it should be. There would be a closely lapped taper fit between the valve body and the rotating core. Unleash the bottom end (opposite the lever handle), removing a split pin, flat washer and spring (as I recall) to extract the core. After cleaning you might use valve lapping compound on the mating tapered bits, and maybe spin it with a power drill (or by hand) for clean-up and seating of the seal surfaces.

These pictures should look more like brass:

It is possible to install a tapered thread pipe fitting in place of the original straight thread drain cock, if you use Teflon tape for sealing and screw it in tight enough to prevent leakage. There is some evidence now that at least some of the Austin B-series engines may have been originally assembled with tapered thread coolant drain cock rather than the "official" straight threads. Not sure if these were perhaps Australian built engines, or if it is just a coincidence that some engines may have been field retrofit with the tapered thread drain valves.

Original straight thread drain cock -- Tapered thread drain cock -- Tapered thread plug.