I have the exact same question. I see your post was from february, and now here it is September, and doing many variations of a google search for tube modules... M265, M395, M515, AND M640 etc. does not generate the results I need to track one down. Perhaps I just haven't found it yet. Of course they must be out there. Nonetheless, if it's still this hard to find, I'd say regulations, limited consumer demand, etc might be complicating the process.... and more likely, when I do find one, it'll probably be the most expensive steel tube I've ever bought.
It's times like this a guy wishes he had a pile of scrap metal tubing laying around, his own machining shop, and the time to go and fabricate something.
I'm also a junior member, new to this forum (but not Benelli)... so if anyone has any info regarding an M515, or M640 please kindly advise.