It's probably a BT-45. If so, it's the only dual compound bias ply tire for the Norton that I know of. I use them on my BMW K75.
I'm using Dunlop GT 501 on my 72 Combat. 19 front, 18 rear. Far superior to the Avon Super Venom I used before. More than twice the wear and handles great. I had problems with an Avon Super Venom rear. I developed high speed wobbles. I went through everything, all to no avail. Finally put a new rear tire on and bike tracked like an arrow. Changing to a 18 x WM4 rear rim was one of the better changes I've made. Greatly increases tire choices.