My car was first registered 25.04.80. From experience with other Standard cars I wouldn't trust cut off dates, especially so close to change dates. SE (special edition) and 'Limited Edition' cars used left over bits to produce them.
Like you I looked around for a clutch and VERY NEARLY took the plunge, had I done so I would now have the wrong clutch kit.
This is the link for Borg & Beck https://webcat.borgandbeck.com/Home
Put in Triumph or Rover to do the search but the kit number for the 7 1/4" kit is HK8917, for the 6 1/2" kit it is HK8930. All else fails give Borg & Beck a ring, I did about another clutch I have.
I have a number of old slightly scored clutch cover that I have taken out of Herald 13/60's, identical to the one I took out of my Dolly, the clutch plates are identical too except mine is 20 spline, the Herald is 10 spline. I cross referenced the cover part number listed by Rimmer with Canley Classics, their site brings up Herald 13/60 + Mk IV Spitfire. The pressure plate GCC197 is under the Unipart part numbers GCC133, GCC134 & GCC168 and Borg & Beck part numbers HE1011, HE2830 & HE3598 as they all cross-refer to QH part number Q10001
The clutch plate is GCP188 165mm 20 spline and also fits the Morris Marina 1.3 plus Triumph Dolomite 1300. I managed to find a pair on ebay for £15 each, from the stamp on them they are 'Borg & Beck approved' so probably ex-BL.
If I were you, I wouldn't trust numbers, I would take your box out and go by what you find, someone could have played with the set up over the past 40 years.
The box I took out was a replacement, possibly 'professionally' re-built, it was nicely painted all over in signal red.