The mystery continues. I received a reply from the BMIHT:
"I have checked the factory record and '2.6 ply tyres' is definitely recorded, it is quite clear. However, as to what it is referring to, like you, I cannot make out. I've been through Bill Piggott's book 'Original Triumph TR' and can find no mention in that either.
Best Regards
Richard Bacchus
Certificate Officer"