The "B" in B1850 means that you are setting up a fully-coupled case (active ocn, atm, lnd, etc). If you want to run an 1850 case with CRUNCEP data then use something like, for example, I1850Clm50BgcCropCru (depending on which version of the model you are using). The "I" here mean you are setting up a land-only case, and forced by an atmospheric dataset (Cru: CRUNCEP).