Honda-SOHC

You dont ever use the slot anyway, when building before fitting cam stick fingers down tunnel, push slipper to carbs and hold whilst nipping up locknut then just undo when built and holding pressure on kickstart, nipping up whilst holding pressure.

Check the tensioner fold over tabs are crimped over tight to hold theteeth on the bolt in mesh with the rack, people forcing the screw slot in the false idea that this will tension the chain is what damages the meshing in the first place

Done. Raised my voice a bit too! 

As long as the nut screws on leave it alone as you dont need that slot

found on mine that you need a magnet to tell if washer is stuck to head on some of the less accessable ones

A top tip Martin; I'll try that, thanks!

Are we talking here about the captive type stud/bolt  that is normally part of the tensioner blade - not available as a separate part? Is your tensioner blade in otherwise good condition.

The tensioner blade and mechanism assembly looks to me to be in good condition. It is just the head of the screw that goes through the 10mm nut with the little gear bit on the end. Where the outer part of the locking screw should have a slot it has had too much force applied with a screwdriver and one side of the slotted part for the screwdriver has sheared off.