It is indeed frustratingly difficult to find specific antibodies for CB1. We tested a panel of commercial antibodies and none detected CB1 specifically; the only ones we've found to be selective were produced by Ken Mackie's lab - see http://www.ncbi.nlm.nih.gov/pubmed/18406468. Even still there is potential for differences in specificity between lots of antibody, so you need to run appropriate controls even if a previous lot of the "same" antibody has been validated.