There is only one single cam head for the RB series of motors and thats the RB30.
The Twin-cam heads were the RB20, RB25 and RB26 heads.
*It is my understanding that any of the above twin-cam heads will bolt up to RB30, though im sure there would be clearance/compression issues with the RB20.
sorry about that last messige i rote it in a rush,, what i meant to say was is there any rb engines that come out with a single cam set up besides the rb30