Since very few norton owners are aware of the different versions of style and thread length, I have put a link to a pix that I also posted on the jerry doe site
This is what the pix shows:
1949-1972 left: D12/167 18092 NM18092 06-2464
SMALL fin LONG thread
1973 750 center: 06-3555 RARELY SEEN OR RECOGNIZED
"BIG fin LONG thread"
These were the ones on sale at BCS some months back. I'm not sure they knew the identity of what they were selling...
1973 -75 850 right: 06-3988.
BIG fin SHORT thread
atlanticgreen.com/images/exhrose.jpg