I think members of the Jerry Doe Access Norton (Norton Commando) website forum
http://www.accessnorton.com/ who have AM 26 Roadriders, usually use the 100/90 size on the rear, either in the original 19" diameter Commando rear wheel size, or 18" diameter if the rim has been changed to allow a wider choice of rear tyres. I think a few Access Norton members experienced some handling problems with wider rear tyres if I remember correctly.
A 2.50 rim size is a bit narrow for a 120/90 tyre, and the Avon Roadrider rear fitment recommendation for an 850 Commando is either 4.10 or 100/90 (in the original 19" size) on a 2.50 rim.
http://www.avon-tyres.co.uk/motorbike/8 ... r-commandohttp://www.avon-tyres.co.uk/motorcycle/roadriderYou can always use the search facility on the Access Norton forum to find more information on this and any other Norton Commando related subject (and why not become a member too?) and here's a recent discussion about this subject:
http://www.accessnorton.com/avon-tires- ... t6122.html