Yep, that's Jos Schurger's BS-based special. He came 3rd in the 1973 125 World Championship on that bike (including winning the Belgian GP at Spa).
According to some random facts I found elsewhere on the internet (some time ago), it was very heavily based on the Van Veen 50cc racers of the day. The frame was custom made for Jos, but it was essentially a bigger copy of a Van Veen. It used a set of BS175 cases with a custom crank and a pair of Van Veen 50cc cylinders which had been bored out to displace 62.5cc. Pistons were by Mahle. It started life with a fairly stock BS 5-speed gearbox but a special 6-speed cluster was built for it by midway through the season.
I'm not sure why Jos put "Bridgestone" on the side as by that stage the factory had stopped producing bikes and didn't have anything to do with developing his racebike. I don't know if he got any kind of sponsorship from BS.
It raises an interesting question - at what point does a bike stop being a "modified XYZ" and become a "one-off special based on an XYZ"? Personally, I reckon Jos could have gotten away with calling his bike anything he wanted to.
