FWIW, the story I've heard is that SU became part of BMC, which was Triumph's competitor at the time. Supposedly Triumph was paying twice as much for SU carbs as MG was! (along with being more affected by strikes, lack of production and so on). So the ZS carbs were to cut costs and get better control of their own destiny. But then when BMC fell on hard times, they got effectively taken over by the UK government, who applied pressure to Triumph to go back to buying SU carbs. (And shortly afterwards, the two companies merged anyway to become British Leyland.)
Piggott gives the change point (to the SU HS6 carbs) as CT62191. But I suspect that is approximate, since the SPC just has a blank.
IMO the HS6 is definitely a superior design to the H6. The float bowl mounting seals and cork jet seals, which are perennial fuel leakage points on the H6, were both eliminated. The center pull linkage reduces the force on the front throttle shaft bushing, greatly reducing wear at that point. And according to several people (including Kas Kastner), they even flow better! I've got a set on the shelf myself (but decided to stick with the original H6 carbs for now).
The SU patents got transferred to Burlen Fuel Systems, who I believe still make complete HS6 carbs.
https://burlen.co.uk/
These folks
https://www.ztherapy.com/products/SUReman.htm can even install roller bearings and seals on the HS6 throttle shafts. They cater mostly to the vintage Z car crowd, but can do the same for genuine SU carbs as well. (240Z et al used an almost identical Hitachi clone of the HS6.)