Hi Mick, I'm pretty certain that the yellow one started life as one of the red MD250's.
My knowledge of the "RS125" is patchy. I find it difficult to locate reference.
If it is the above case, there is no denying what a magnificent presentation the builder has done to convert it to the track
The yellow one is a 250. It has a base flange on the cylinder.
The 125s run studs all the way through to the head.