These machines are mentioned on EARLY RACE DEVELOPMENT pages, but they could not be omitted from any history of Yamaha at Daytona.
After the 1958 Catalina Grand Prix, Yamaha maintained a presence in the US by sponsoring local riders racing out of their base at Marty's Foreign Motorcycles in California, as mentioned on the CATALINA GRAND PRIX pages of this site, mostly competing in the West Coast. It wasn't until 1961 that Yamaha sent over another team of new machines and riders from Japan, this time for the US GP which was to be held at Daytona in February of that year.
The new machines were the direct descendants of the Catalina YD machines, with the engines code named YX24 250S and the frames YX47, they together made up the machine known as the RR250.
Initially, the works machines were produced with frames almost identical to its disk valve predecessors, which were a development of the YD and first seen on the YX18 (Yamaha's first dedicated road racer, which only made it to prototype stage) and RD48. The RR250 machines featured large alloy 2LS drum brakes and were also fitted with fairings from the YX18. There was no provision for a rear guard (fender), but a front was available although not fitted for the Daytona race.
Engines were YX24 spec Asama racer units based on the 5-speed YDS1 engine. The cylinders were now alloy construction gaining a weight saving over the older YDS1 Clubman Racer engine but pistons still retained 2 rings at this point. Ignition was by Hitachi MC-2RY magneto and carburation was provided by 2 x Mikuni-Amal 276 27mm carburettors mounted on long alloy inlet manifolds with float chambers bolted the rear of the engine via anti-vibration mountings.