Katie, I forget if you have the instructions for the probe. If you don't, I can scan them in.
But you should be able to easily calibrate it for either paper. The manual I have says it was calibrated for Multigrade II paper, but doesn't say FB or RC. You are supposed to measure a shadow area which you want to retain detail, according to the manual. To calibrate you basically make a print that looks good and note the time. Then place the probe in a shadow area (bright part of the image). Then take a reading and adjust the dial until the taken reading matches your known good time. They warn that the probe may need to be calibrated for each batch of paper. My experience with other enlarging meters says this isn't usually the case with Ilford papers, but between papers there is quite a variation.