I have a 3 imperial gallon tank although I don't know what year it would have been, probably later than 67 and there isn't any holes for the rack.
'67 was when Triumph changed the C-range frame and fuel tank fixing to be similar to the 650's - frames look similar and the tanks use the same 3-point fixing (two at the front, one at the back, although C-range tanks are still shorter than B-range). So any '67-on tank fits any '67-on frame but won't fit a pre-'67 frame nor will a pre-'67 tank fit a '67-on frame.
As Chris posted, '68 still lists the tank rack for UK & General Export tanks, '69-on doesn't ... although I could mess with eveyone's heads by posting unqualified that I've a '73-on (no top seam) 3-gallon tank with Cycle-thread holes for a rack ...
in fact, it's a police-bike tank, the 'rack' mounts are for a radio ...
I have photos of five original-paint tanks, only ONE of which has the screw holes for a tank rack.
The one with the screw holes is definitely a US-market tank?
and Triumph Tiger 100 And Daytona
echo what Chris posted, no racks on '67 US-market tanks. None of the '67 US brochures or ads. on classicbike.biz show racks?
Aiui, the decision was made to leave racks off US-market tanks after Triumph(?) was successfully sued by a rider who slid up the tank during a collision and bashed his 'nads?