These machines are mentioned on EARLY RACE DEVELOPMENT pages, but they could not be omitted from pages about Yamaha's history 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 USGP which was to be held at Daytona.    
The new machines were the direct descendants of the Catalina YD machines, code named YX24 250S and also known as the RR250 (although teh works specials differed somewhat to the production RR250 that followed). The frames 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. The RR250 machines featured large alloy drum brakes with a 2LS front and SLS rear 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 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. 


Fumio Ito lead the team with Yoshikazu Sunako in support, both veterans from Asama and both would make up part of Yamaha's team in the 1961 World Championship Grand Prix. It was of course Ito's 2nd trip to the US after leading the team to at Catalina. 
RR250 Daytona 61 sunako.jpg
​Ito rode the #3 machine and Sunako #4.  They finished a very creditable 7th & 11th places in Yamaha's 1st official road race debut on US soil.
The special RR250 machines were not the 1st choice for the Daytona race as Yamaha were hoping to run the new RD48 disk valve machines, ahead of the World Championship races soon after Daytona, but these were not ready in time.  Instead these prototype machines were to be used as demonstrators for the forthcoming production racers, which were soon to follow on from the YDS1 Clubmans Racer.
US Yamaha development riders and works supported riders such as Tony Murphy and Dave Buising were given rides on the machines in private testing sessions, and the bikes received much praise over the previous YDS1 Clubman racer.
"I had a chance to try out the specials that the factory had down at the International at Daytona" wrote Dave Buising to fellow YDS1 Racer Geoff Kellond,  "They had beautiful brakes, double leading shoe front and rear, and were very light. Weighed 192 lbs with fairing." he went on.
The Daytona machines were essentially works prototypes ahead of the official very limited production RR250 model which were marketed in very limited numbers, 10 across the entire US, soon after the event.