Honestly could not tell you. I started with the 1983 Red Box Basic, so no experience with B/X which was retired by that time (well, replaced with Blue Box Expert). Yes, the Blue Box came with The Isle of Dread included. Not sure if there were any changes to it, but it was a formative experience.Wasn't X1 the module included with the X of B/X?
Edit: Was the updated version with E of BECMI much different? (It was packaged with it too?)