Just one point... this has been like that since the introduction of R classes. It's not the number but the letter what represents the engine. The number represents allowed level of tuning. R2C and R3C have always had same cylinder capacity just to my knowledge no manufacturer ever homologated R2C car. Same case is R1B and R2B where both are up to 1600 ccm but R1 is stock car while R2 is reasonably more advanced one.