The RFS head standard has retainers at top and bottom of the valvesprings. We have purchased a catcams kit with solid lifters and retainers/springs.

Is it necessary to use bottom retainers? The innerspring inner diamater of the catcam spring is bigger and doesn't fit properly on the standard bottom retainer. As far as we know there aren't aftermarket bottom spring retainers available.
The RFS head is the first I have seen to have bottom retainers, would like to know the reason behind. If I were to guess it would have something to do with the double spring configuration?

The innerspring dia of the original is 13mm and the catcams innerspring dia is 16mm.