The buckle shown, does not belong (and never has been!) on a Stable-belt. It is much too heavy. It is often also seen with the King`s Crown, as the GS buckle is that ancient! There are two different areas here, that have been wrongly combined.
The belt is a normal belt of the Royal Artillery, minus it`s leather accessories or link buckle (Army cadets are renowned for constructing such mixtures when parts are scarce).
The wearing of Stable-Belts (a non issued Item) with Dress uniforms is and was, strictly against Army Dress Regulations. That means, there are two forms of Dress here also. A unit RSM would have a fit. It is also badly in need of a clean.