Fred, as far as the base metals go, this might be true.... (Is the second base metal included?  I thought it was P1 - P11 and don't recall p15b being included?) HOWEVER, just make sure you triple check all of the other essential variables too because sometimes things change... like purge gases just as one example, filler metal may be another.  Just make sure you've got all the essential variables covered and nothing changes from similar to dissimilar welding.  Have fun along the way.

hello, welder qualify with P1 to P1 is qualified to weld the P15f inclusive.
regards