Hold on - slow down a bit!
"The bars that were with the remains of the bike when it was retrieved from the scrap heap are obviously not the originals"
They may not be the originals, but given that they were in the same scrap heap perhaps they are. It's interesting to ponder why a set of bars from a completely different bike would be together with the almost-complete remains of another.
I know that SDs used rather "daggy" bars on some bikes, as a few years back we had a very original SD in our club (no longer here) which was fitted with these rather old-fashioned bars. There are various bars listed and illustrated in the parts list (see pages 19, 22, 23, 24), and some are fitted with "Magdyno Lamp Bracket". Are you sure that your bars are not one of the options listed as "Handlebar (Touring Pattern) complete, with "Magdyno" lamp bracket"? Before you throw them away, you might want to think about how you KNOW that they are the wrong bars.
By the way, attached is a photo, from the Yesterdays Archive
http://www.yesterdays.nl/triumph-1925-model-p-710.html showing a drum-brake SD. The brake cable exits the handlebar exactly where your "mystery thread" is located, and I fancy I can also see the cable guide 2828 screwed in to said thread... The exhaust lifter cable exits through a hole drilled in the handlebar - perhaps someone brazed this up on your bars prior to plating? (Try not to rely on photos of restored bikes if you can avoid it...)
Cheers
Leon