Some SU carburetors have a threaded port for connection of a vacuum pipe for ignition/distributor vacuum advance. For MGA and early MGB this port is on the under side of the rear carburetor (of dual carburetor setup) just below the throttle valve plate. Sometimes the thread in the throttle body is 5/16" BSF, 22 tpi (see exception below). For the original nipple, thread on the other end is 5/16 BSI (British Standard Iso - 26 tpi). This is an unusual thread stemming from BSC (British Standard Cycle) where threads over 1/4" are all 26 tpi. By coincidence, 8mm x 1mm pitch is almost identical (within a couple thousandths of an inch) so an 8x1-mm screw plug or adapter would probably fit here.

For a bit of speculation, I will guess that threads on the other end of the vacuum pipe (connection to the distributor vacuum unit) will be the same 5/16 BSI, so the vacuum pipe could swap ends and still connect.

For more fun, there are four more "odd" threads in the MGA carburetor.
See off site video - SU H4 Carburetor Threads (MGA) - University Motors, Ltd.

Thanks to Mike Ellsmore , this drawing of the part was procured from Burlen. Click for larger printable PDF image file.

There was a change of thread here. The early MGA 1500 (first 11,200 engines) used the 5/16-BSF thread, where later MGA used 7-mm thread [0.276-inch] in the throttle body and no adapter fitting.

On Jul 23, 2017, Russell Goebel in Wynnum, Queensland, Australia wrote:
"The best fit was 7mm screw (bolt). I am sure the correct thread is NOT metric, but this was the best possible fit".

On Jul 23, 2017, Jonathan Eagle in Weston, MA, USA wrote:
"The Moss vacuum cables are designed to fit into a 1600 carb. The 1500 carb had a larger threaded hole. Both of these fittings are available. You could solder closed a piece of tube to get what you want".

On 13 August 2017, Ray Watson in Victoria, Australia wrote:
"I have one of those nipples and it does not screw into the body - much too wide. My vacuum advance unit goes directly into the body with a hollow screw and a small "acorn" fitting around the brass tubing".

On 22 August 2017, Ray Watson in Victoria, Australia wrote:
"There must have been a change in the thread at some stage. I have about five 6020 bodies and the union you pictured is too big for all of them. .... I have located a spare male fitting that I can block with solder that does fit. Thread pitch is 1-mm as near as I can tell. Problem solved but I am still curious. I have included an image of the union, plug and male fitting".

That's a compression fitting around a piece of tubing, so it shouldn't be too difficult to get a piece of solid brass or copper rod the size of the tube, and install it with a new compression fitting, no soldering required. Or squash the tube.

On 09 September 2017, R Watson in Victoria, Australia wrote:
"I have found the correct thread with help from Joe Curto. It is 7mm x 1mm and appears to fit perfectly. Not a common metric size and a bit difficult to obtain".

And we have a threaded plug for this port, part number AUC-1289 Plug, 7mm x 1mm.

On Feb 24, 2024, Ron J in Thunder Bay, ON, Canada wrote:
"Problem solved with a custom made adapter
(made by an SU enthusiast in the USA).
M7 female x 5/16 22 TPI male. Aluminum.
This allows the use of the existing newer style vacuum line with the older type of SU carburetor without modifying the vacuum line".