Benelli m2 and Remington 870

What size nordic components Extension is on this benelli.
Im certain this is the benelli m2 field with 21" barrel(lol, because further down the page he says it is…).
-The picture posted by apdswat.
http://tinyurl.com/cyoe4ru

Also, a +3 extension from whoever is even with a 20" remington police barrel right??
If so, from where should i get said extension?

Thanks

Looks like +5