650ss Front Brake Cable Mudguard Steady

You need Bacon's 'Norton Twins Restoration'.  Lots of pictures.
The clip is spot welded to the mudguard.  If you don't have one, you will have to improvise.

Its just a shaped and bent strip of metal which could be spot welded or riveted in position. Its likely that the guard is a pattern or incorrect one. Brake cable type will depend on front brake type , single and TLS are different.

Sounds like another cable tie coming into sight!!!

If you go for a p-clip and pop rivet remember to size it so the handlebar nipple can fit through. It will likely be a floppy fit over cable outer. Original clip can be bent a little to fit cable under open part of clip, and then closed again. 

... you could fashion something from stiff wire (I used a wire coat hanger as donor) and secure it to a mudguard mounting bolt on the slider.

Attached is one I did for Doris a couple of thousand miles ago.
 

The short roadholder Manx had a similar wire guide to keep the cable from flapping around (somewhat longer loop).  What is important is that you don't restrict the movement of the cable when the forks are depressed under braking. The brake feel stiffens if it's too well constrained.
 

The Nomads used a very neat front brake cable guide that was attached to the stud on the front fork slider. It had a small gap that the cable went through, to keep it in place.