The bike was designed using the XL250S (1978) twin exhaust motor.
(later in 1985, JJCobas from Spain applyed same solution on his revolutionary bike)
Here you can appreciate the clever intermediate chain System, from J.F., Spain, still under restoration.
This second chain comes from gearbox sprocket to countershaft mounted on the swinging arm pivot. The large chain comes from this swinging arm pivot to the rear wheel.
George Silk engineer from UK also make some studies talking about same chain concept.
See following Youtube interesting video.
This is s short clip looking at trials riding and features some very early hill climbing and Mick Andrews on his Ossa at the annual Scott trial in Yorkshire. It then goes into featuring George Silk who had a new concept for a trials gearbox and swinging arm
More info about Fraser HT250 at: http://www.onlytrial.com/from_1974_35.html