Not sure about the RDM, but my 5.5 ncx rigs on a NP X6 SDM...not a perfect match, but went pretty well today!
...Actually, I've gotta be totally honest...I used it on a X3 base (430) with a X6 top (460) with a shim made of an empty silicone tube, cut to length, sliced down the side and slid between the connection to pack it out!
30-40kt winds! Desperate times call for desperate measures!