Background
Up to the mid-1960's, in line with most of the rest of the British manufacturing industry, Triumph and its component suppliers used British Standard threads. In 1965, the British government announced than the UK would be metric within ten years. From then on, the British automotive industry mostly swopped to Unified threads (as the whole industry became more and more dependent on exports to the US) ...
Afaict, Triumph started fitting rear wheel speedo drives around the same time as its first unit-construction engines, so the 1957 model year for the 350 "21" (although the 650's continued with gearbox-driven speedos for a couple of years after the engines became unit from '63), certainly long before any change to Unified threads. Rear wheel speedo drives were supplied by Smiths, same company that supplied the speedo and tacho heads and cables - Smiths made dial-type instrument for a wide variety of other industries nothing to do with motorcycles, cars, etc. (my first wristwatch was a Smiths - didn't keep very good time and although allegedly self-winding, it needed hand-winding once a month ... Seiko I bought in Singapore for £5 was kept better time and didn't need the hand-winding
).
In line with the rest of particularly the British automotive industry, Triumph began to change to Unified threads from the late 1960's, although it took 'til the mid-1970's to either change or eliminate components with British Standard threads, why most of the threads on your '79 are Unified. But not all of them ... Triumph's component suppliers like Lucas and Smiths were slower to change or eliminate components with British Standard threads, particularly as the British motorcycle industry contracted through the 1970's, requiring smaller and smaller quantities of some specialist components from suppliers. Is one reason the fuel taps and the Smiths speedo drive even on a '79 Triumph are still BSP threads (the taps aren't any of the other thread types
@Greyfell posted, I'd be very surprised if a Smiths speedo drive grease nipple is either).
However, what stops BSP threads being obsolete along with other British Standard thread types is somewhere along the line, the ISO adopted BSP as a standard specifically-pipe thread,
in preference to US National Pipe or very fine-pitch metric threads.
Why would I not want something that makes it easier for me?
By "[if] you really must have a "90 degree fitting" and nothing else", I meant if you wouldn't consider any other alternative "to make it a 10 second job of greasing the speedo drive"; e.g. I have a grease gun with a long steel pipe to the nipple fitting, the pipe is long enough to reach a Smiths speedo drive grease nipple from below a rear disc caliper.
From
https://www.machinerylubrication.com/Read/584/grease-gun:-
The Zerk design, named after Oscar Zerk, used a fitting much smaller than the Alemite pin-type
... however, note:-
and did not lock the hose coupler or hand gun and fitting together. Instead, the seal between them was maintained by the pressure of a pushing action when the operator applied the coupler to the fitting.
I am not sure if this is a Zerk NPT pipe thread 1/8"-28 screw in fitting and a BSP nipple.
"Zerk" applies to the nipple form (as opposed to "Alemite" on that "Machinery Lubrication" website page).
Otoh, "BSP" is a threadform - 1/8" nominal pipe diameter and 28 tpi (threads or turns per inch)
is 1/8"BSP, 1/8"NP (NPS or NPT) is 27 tpi and, as I posted earlier, the thread has a larger o.d. than 1/8"BSP. As with "zerk", you can enter "bsp thread", "npt thread" into any internet search engine and it should return links to tables showing dimensions like o.d., tpi, etc.
difference between a BSP grease fitting and a BSPT grease fitting?
As I say, BSP - British Standard Pipe - is a threadform (group of threads of different diameters but having certain common features).
By standard, "BSP" is specifically a 'straight' or 'parallel' thread - same diameter all the way along. Otoh, the "T" suffix in BSPT stands for "Taper" or "Tapered", meaning one end of any given thread is thinner than the other end - given a great enough length any "Taper" thread would end in a point. Tapered threads are often used where the connection transmits a liquid and speedy coupling/uncoupling is required; a tapered male thread will engage a female thread further 'in' and more threads will engage at the same time; similarly when disengaging.
As an aside, the oil pressure switch in the front edge of your bike's timing cover is 1/8"NPS - National Pipe Straight.
