Reviving this old thread.
So you are saying a Nova extension tube would work on the M4? I have the H20. Would it work on that also?
https://www.botach.com/benelli-80201-nova-supernova-3-shot-magazine-extension-kit/
Found this one searching the part number you listed. 4 times cheaper.
Also, has anyone tried having the tube be longer than the barrel? I saw some guy on a YouTube video with a 12 shot. Looked to be about 3" extended past the barrel.