Compared to your photo of the RCBS die plug, and my photos of the Lyman M-die... the differences are obvious. The RCBS in the photo appears to inside neck size with the additional ability to flare the case mouth, the M-die inside neck sizes with the additional ability to 'step' the case mouth out slightly for ease in bullet seating. It all depends on what you want.
My RCBS Cowboy die set has the M-die style plug; I don't know if it has the ability to actually flare the case mouth... I never ran it down far enough to know, but now I'm curious.