Most of the mainstream gang mowers would have cylinders that would disengage and were all painted green in one shade or other.
As you say, Ransomes, Lloyds, Shanks and Greens were the common makes and they may have painted them yellow to match the rest of the local authority fleet. We used to have to spray Ransomes 213D’s yellow for Coventry City Council in the 1980’s.
Have you thought that with the yellow colouring they may have been Sisis- not many about and not certain how many were made but definitely a possibility.