Norton's very successful Manx single cylinder racer provided the styling inspiration. Featherbed Rear Frame Loop - Norton's 'Featherbed' is without doubt the.
Click to expand.There is a reference on atlanticgreen.com. Earlier frames had a narrower rear lope while the 850 frames were wider. I'm surprised you are having a problem with the FB tail section fitting a '72 frame though. I had no problem with my '72 when fitting a '68 tail section. Narrow is 6 3/4' inside the frame loop 4' forward of the rear of the loop. Wide is 7 3/8'in the location.
![Norton Commando Frame Loop Norton Commando Frame Loop](/uploads/1/2/5/4/125429048/895020635.jpg)
I have a bent 850 frame in the garage. It has the wider loop while the '72 frame has the narrower measurement.