Stable belts were not designed or manufactured by the MOD. They were bought in bulk, very often from an Indian or other Asian source (they are still some of the best in the world at heavy weaving!) by the Officer who ran the PRI Shop at the time.
The PRI kitty is the “private” money of the Regiment. Belts only had to be approved by Dress Regs (many were not, and were approved by the CO only! An example of this are the RHA Battery belts). The woven fabric remains the same, per Regiment, as it is “running feet” material from the roll. That leaves the choice of buckle only.
There were and are no specific buckle types to a type of unit. Cadets or Training or otherwise. There were no difference between stable belts and different Ranks, as the buying source was the same. However, there was an evolution of buckle types (although here, the PRI officer still had the last choice).
Herewith a rough timeline:
The original buckle was the single tongue leather buckle (as in the old sand coloured GS belt around 1900):
(I wore this buckle with The Royal Sussex Regt in 1964)
The second version was the wider, double or multi tongue leather buckle (around 1920) which originated from the “Horse Regiments”:
The third version is the one adapted direct from the 37/58/95 pattern Web Belt clasp and appeared in the early sixties:
(I wore this one in 1962 at AAS Arborfield)
The next version within this “evolution of buckles & Clasps” was the one which was adapted direct from the old white buff Dress belt, which originally carried the GS buckle for all units:
So THIS: (originally Victorian)
Was adapted per Regiment, like THIS:
This was around the middle to end of the seventies. They still seem to be popular today.
Running almost parallel to the above from a date point of view, was the adaption of the rectangular Chrome plate and Cap Badge design.
(which I wore in Queens Regiment)
This was adapted directly from the New, white dress-belt made of cortelene, on which it appeared first. Today, many Regiments use the same buckle for their Dress Belt and their working stable-belt.
The latest addition (started to emerge in the eighties) are the modern clasp versions, which are much lighter than their GS type forbears and are designed with advanced optical quality.
Having explained the progression of fixing the stable-belt from the date aspect, we must add, that of course, all versions are still in use, irrespective of age. As long as they remain privately purchased items, design will apt to wander and will remain a matter of taste.