Julian, thanks again.
I am fairly clear looking at your examples my box marked STD is not in bad condition, by slight rounding I meant profile of the teeth not the dog itself. I have priced the parts from Draganfly and have 2 further questions. I think my layshaft is machinable but for longevity, I will buy new.
However, Draganfly provide a layshaft and specifically say it is one without the fourth gear attached. I have not TRIED to remove mine, but it seems fairly rigidly attached. Are there two versions or do I just need releasing fluid and elbow grease?
Second question, it is the clutch end of the layshaft that has much worse ribbing. Has anyone tried a mix of the close ended needle bearing, including a thrust washer, in the clutch end and a brass bush in the inner cover end? this would seem to resolve the ridging issue whilst negating machining. Is there an issue I have not thought about? If any further work is needed, like the other needle roller bearing, I will be able to do it with the box still in the bike.
Finally, I will of course replace both mainshaft bearings, it would be remiss not to.