You are correct about the sequence of carbs: HD8s from 1966-70; HS8s from 70-73; and HIF6s from 73-end.
But the HD and HS series carbs are noticeably different. The dashpots, pistons, butterflies etc are probably interchangeable, but the float arrangement is not the same at all.
The HS type (as should be familiar to anyone who ever bought BMC) has a separate float chamber bolted to the
Side (hence 'S') of the carb body, with a flexi hose delivering fuel to the jet, which is mounted underneath. The needles are biased.
The HD type also has a float chamber on the side but it is mounted on a cast-in bracket from beneath the carb body. A
Diaphragm (hence 'D') is employed below the jet, which is contained within this housing. There is also something approaching a Swiss watch factory's parts bin of springs locked away inside for reasons that aren't immediately apparent! AFAIK, the needles are also fixed.
The pics on Burlen website show the difference externally. The emission inlets, which can be seen on the HS series, are absent on the HD.
http://www.sucarb.co.uk/ProductCategory.aspx?ParentId=1
Apparently, the reason Rover moved from HD's to HS's for the TC was solely to gain the emission inlets, which were becoming a requirement in the late sixties. Although undoubtedly the simpler operating principle of the HS series was also of considerable benefit in aiding adjustment and servicing.
Those in Greg's picture are definitely HS8's. Aside from the emission pipes, the fuel lines enter the top of the float chamber through a little cast-in T-shaped branch. HD's have dome-topped chambers with fuel lines entering on the front.
Michael
PS: I definitely don't purport to be an expert in the matter..! I'd gladly hear any other interpretations.
PPS: Didn't know that about the rads, thanks!