After posting this question to this forum and initially receiving no responses, I emailed Benelli and this is what they told me:
"...if you take the current M-1 barrel and place it on
the M-2, you need to make sure that the forend from the M-1 goes with
the barrel; think of them as a matched set."
Seems odd to me, as I can't understand the rationale for keeping the barrel and M-1 forend together, but if this is what Benelli says, then...
Batman