Sorry, its not a Scammell, its a Tasker of Andover 6-8 Ton Ball coupling, about 7000 were built, there is an article in the June 2002 Classic Military Vehicle magazine, the pic you used above with the clock tower in the background is the 1st page of that Article, also includes the specs etc.Do you already have the magazine ? If not let me know and I can scan/email it to you although there is no clear large image of the coupling.
Interestingly the Trailer is permanently connected to the coupling and with limited travel of 20 degrees ended up rolling when not on tarmac and a lot where scrapped.
Hopefully I have successfully attached an image of one from Google Images, however if it does'nt then search Google Images for Tasker coupling, or bedford coupling, hope helps !
