I've done a little more digging, but only because I bought a NOS dash panel from Rimmers that had the correct square hole plus 2 round ones that I can't use with my original wiring, and I wanted to know what it should be fitted to...
Original Dolomite up to WF27181 (RH manual) / WF26608 (RH auto) / WF26610 (LH):
These cars have the rectangular hazard switch.
They also have an adjustable "panel rheostat", or dashboard dimmer and need a round hole in the dashboard.
Cars fitted with tandem brakes have a warning light on the dashboard and need another round hole. (This one is like my Rimmers dashboard, but my car is a little later so I use the extra holes for something else)
Gauges are the original black rimmed type, and the walnut is darker than later cars.
Dolomites were produced this way up to June 1973.
Dolomites from WF50001 up to late 1975 (and the first Sprints up to VA10745):
These cars still have the rectangular hazard switch.
The new "3mm pin" harness is used, and the rheostat is not fitted anymore or since by Triumph*. 1 less hole in the dashboard.
Cars fitted with tandem brakes have a warning light on the dashboard and this needs a round hole.
Gauges are the peripheral vision / black and chrome rimmed type, with the more common lighter walnut veneer.
After that (series 3 Dolomite / series 2 Sprint) the move to round hazard light switch happens, bringing with it a dashboard mounted heated rear screen switch (another round hole, but the switch is initially small and round, later rectangular) and a seatbelt warning light (1 round hole plus 2 smaller round holes either side. Again, tandem brake models get a warning light, and another round hole in the dashboard.
*I thought about retrofitting a rheostat to my car to occupy one of the holes (the other now has my heated rear window switch as the original under dashboard type is missing and I can't find an original). It's possible if you amend the loom, so perhaps some cars have this retrofit to add to the confusion? I abandoned the idea and fitted a period rocker switch instead to isolate the radio.
PS my Dad lives in Perth and he once owned a Dolly - but this info did not come from him: he's not a fan and remembers too well the low skill levels of BL mechanics back in the day and switched to Toyota long ago