These wide belts and unusual sliding buckles appear to have been worn by orderlies and mess servants in the British Indian Army. Most of the belts shown have not yet been identified as to which Regiments that they belong to.
I've always been puzzled by the wide stable belts with the 'unusual sliding buckles'. If memory serves me right (my belts are in storage boxes) I have one in RAMC colours and one in RE colours. They look early and I wondered if they were pre WW1 just because of the look and feel of them. But.......I have just received a copy of the booklet sent out to subscribers of the Indian Military Historical Society. There is an article about the RIASC with a group photo, it's not the greatest picture, small, b&w and not a close up, but it shows two or three Indian soldiers wearing what at first glance looks like Regimental cummerbunds but when you look again there appears to be some sort of clasp and a buckle to one side similar to current stable belts. The belts also appear quite wide, and although mostly showing RIASC you can also make out a couple of different belts in the group.