Hi Roy, I don't know if Andrew (Priory) is open for selling parts at the moment but have a look at his list of speedo cables. If he's not open you'll find the info you need there. I bought one last year from him for my 1960 GF s/arm with a chronometric. Fits perfectly.
https://sites.google.com/site/priorymagnetos/cables
Go to bottom of the page and in red 2nd table up from bottom. (Edit) Table heading is Valve Lifter??? but it's for BSA speedo cables & it also specifies the ones that fit Triumph.
Hope it helps.
That's the one Roger, 3'8" also fits Triumph.
To fit B31/B33 (1958-60), A7/A10 (A10RR/A10SR with cowl) (1958-62), A50/A65 (1962 on).
Triumph - 5T/T100/6T/T110/T120 (1954-62), TR6 (1955-62).