WSII and WSIII use the same size tubes but different bushings. It should not be a problem to use pre-tubed WSII blanks on a WSIII pen, but the opposite might not work.
Some pre-tubed WSIII (aka Sierra Vista) are intended to take advantage of the larger pen size to leave more meat on the blank. If you try to use them on a WSII, you might wind up cutting into the embedded object.