This Motorcycle appeared in an Auction by Bonhams Auction House at the Imperial Hotel and Casino in Las Vegas, Nevada USA on 6th January 2011 and was sold for $10,530
The below details have been copied from the Bonhams online catalogue entry which can be seen at this page.
191A• Ex-British Trophy Team
1968 Cheney-Triumph 500cc ISDT
Frame no. MK2 T6
Engine no. T100SS H20991
The International Six Days Trial (ISDT) was accorded great importance by European manufacturers. Until the mid-1960s machines were often ‘street-legal’ motocrossers, or carefully prepped stock highway models with knobbly tires; inevitably, the ISDT results had a significant impact on sales. Selection for one’s national Team was highly coveted, likewise the winning of an individual gold medal. Various nations have dominated the prestigious Trophy and Vase Team awards; Britain until 1953, after which their heavy 500/650cc machines were outpaced by Czechoslovakia and East Germany on home-produced 2-strokes. USA won the Vase in 1973 and 1991 but the ability that so frequently wins the MX des Nations has not yet been attained!
By the late sixties a loss-making British industry was no longer able to supply their national team. They thus turned to the brilliant Eric Cheney, whose BSA and Triumph-powered motocross machines were winning at the international level.
Eric built a handful of dealer-sponsored ISDT Cheney-Triumphs; ironically, some riders preferred his to the factory-supplied machines used previously! With freedom of choice Cheney chose the best equipment for his own twin down-tube chassis into which was slotted a Triumph 5TA motor. Together with tapered conical hubs, Ceriani type forks, and larger alloy fuel tank – but with lights and muffler added – the machine was a highly competitive package.
This ex-ISDT machine was used in the 1968 British Trophy Team (rider unknown), but apparently retired on the first day. Purchased by the vendor from a respected British dirt shop in 1969, it is in authentic condition, with the added bonus of a 1982 restoration by Danny Macias, former BSA/Triumph race chief. Truly, a pedigreed machine with historic provenance! Sold on a Certificate of Title.
Speed Track Tales 
This bike was ridden by John Lewis and he retired on the first day due to a broken rocker. The brand new engine was fitted into the Cheney frame by Arthur Bates. The frame is painted black not plated as a centre stand, various lugs and seat hinges etc had to be welded on and there was no time to get it plated! Oil is not stored in the frame as was usual for Cheney motocross frames but in an alloy tank with filler under the seat hence the need for a hinged seat.
The original silencer was off an Enfield trials bullet and fitted neatly along the left hand side.
The bike was sponsored by Tim Pritchard of Westbury Motorcycles of Westbury on Trym, Bristol who supported John’s motocross career for several years.
Tim originally supplied 3 frames and engines for John, Arthur and Billy Mills, but due to Billy’s tragic death earlier in the year only John’s bike was completed.
Thanks Geoff I will update our post. It is really kind of you to take the time to share.
loved the article,,and have a question,,i have a older cheney triumph that had a replacement engine in 1970, would this have been a standard t100, the bike is a american import, and i am trying to find out who tuned it,,, it goes very well for a 500. unfortunatly the previous owner is no longer with us, so i was wondering if it was tuned by triumph or the american importer,, one day when i pull it apart i may get some clues,, thanks adrian rouse
Bit naughty of Bonhams to list it as Ex British Trophy Team !!!!