Most of the Triumph specialist parts places will have the correct plug for that hole in stock, E2615 was the early part number.....can't put my hand on my supercede book to tell you the current number though.
You can make a take off of the thread using body filler and a bit of care. Then you can measure the exact pitch of thread. What I gathered from the thread I posted, is that there are 3 possible pitches depending on the exact year of your side cover, 26, 27, 28. Determining the correct pitch will get you well on your way to finding the right fitting.
I was converting a pre unit timing cover to feed side oil filter and like everyone i thought it was 1/8 bsp and ran a 1/8 bsp tap down the hole , it removed a fair bit of metal so i doubt its bsp, but it works now.
i bought a plug for this and thought it was right..... i couldnt even get it to start in the hole. So since I changed my tranny pushrod to the ball bearing between type with new allen adjuster. I tried the old flathead adjuster screw, screwed right in.