Steering Lock

which type of top yoke?

We used to use a lot of roll pins in agriculture. If you cannot find it under a Norton part no. Is there an agricultural dealer near u you could take the lock to get approx size.
. When fitting ensure the thrust is against the split to spread it, or it will fall out. Think about it!
  Buy several
Hope that helps
Tim S

On my 74 Mk2 850, there is no roll pin, or hole to put one. Lock is retained in the top yoke by friction/gravity. (And a little bit of glue at the moment.)
 

Roll pin only fitted on 850 mk3.

I didn't like the stumpy little roll pin used for the Mk3; awkward to fit and crude. I had to remove a keyless old lock by drifting it out, thereby shearing the roll pn. I tapped the roll pin hole with an M3.5 thread and fitted an Allen grub screw with thread locker.  The grub screw works perfectly, so if there is any concern about retaining locks on earlier models, perhaps drilling a hole in the yoke and fitting one would be a solution?