Well, the single seat COIN versions built by NAA were designated T-28Ds as a US version for operation in South Vietnam and the Congo/Zaire. Some COIN aircraft were further designated as AT-28Ds as attack trainers. The COIN versions carried bombs, rockets and gun packs under the wings. They also were used by/exported to Bolivia and Thailand.
The Sud Aviation licensed for conversion/production T-28s were in fact designated T-28Ss. That is what should be used for any of the 245 Sud Aviation T-28 Fennecs, of which 170 were converted, the rest new production. The Sud Fennec was used in Algeria and in the Argentine Navy.
Registration problems exist for warbirds due to different country of registration requirements or owner submittal inputs. Our US FAA seems to register whatever the owner wishes to call it, except for obvious fraudulent submissions. This has led to variations of designators from the original intent named. Trying to change wrong terminlogy registrations is expensive and discouraged by the FAA. Their typical reponse is "You don't want to go there!"
NAA also built three prototype YAT-28Es, nicknamed Turbo Trojan with a 2,450 shp Lycoming YT-55L-9 turboprop engine, first flying in February 1963, with death of the test pilot in the first the next month on the 13th flight. No production by NAA of this variant ensued. However, Aero Industries Development Center produced a turbo version similar to the YAT-28E powered by a different license-built Lycoming turbo for the Chinese Nationalist Air Force. Fifty were built between 1976 and 1981.
Hamilton Aircraft Company built T-28A conversions for civil and military uses, with 1,200 hp Wright 704-C9-GCI engines as NORMAIR R-1 for aerial photography and the 5-seat NORMAIR R-2 with fixed canopy and side door. The R-1 was certificated by the FAA in September 1958. The Brazilian Navy purchased six R-1s with tailhooks in 1962.
All this leads to my question. Should we really base designators on different country registrations/re-registrations for the same airframe? The T-28S with its initial designation deserves to be called that irrespective of subsequent registrations, my opinion.