Hi, not sure if you seen this post from the Benelli forums, but this is what someone over there measured the tube as being. Might not help at all, but I remembered seeing it before.
"The*factory full length tube I have here looks to have a nominal 1.00" O.D. tube with a square thread of 2mm pitch. The crest seems to be 0.9mm (.035"), the root seems to be*1.1mm (.043") and the thread depth of 0.4mm. Again a shallow thread that is covered with a rough parkerization, so those*measurements*are as close as I can figure for now."