I have just removed one from one of my spare carbs which I am sure is original and not been messed with. The circlip in in the middle or third groove and is marked with the number 18234.
These carbs came from a ex courier hack with 72k on the clock that i bought for spares in the late 80's. You can see a slight narrowing of the needle where it would contact the top of the emulsion tube at idle. What killed this bike was a small washer embedded in the side of the piston and deep scores in the barrel. The top of the combustion chamber looked like someone had taken a chisel to it.
The Haynes manual also says the third groove.
The way that I noticed my problem was to insert the needle into the emulsion tube and try to wiggle it side to side, it was clear that one tube had much more clearance than the others. Closer examination revealed that the chamfer just inside the top of the tube had been eaten away and the inner diameter was enlarged.
From what I have read up on these bikes is that they always ran a little on the rich side at idle to workaround the drop in vacuum when you open up the throttle at low revs, and as I found out with my bike, wear in the needle/emulsion tubes can make it too rich when rolling on the throttle at low revs.
Problem is that the needles are a sod to get at so I'm leaving mine as it is for now and sort it out over winter, It's not too bad with a matched set of worn tubes and 10x better than it was with one completely duff tube. It does stumble a bit at 3000 if I'm very gently on the throttle in 6th, This could be carb balance because they have only been done statically, I put my vacuum guage adapters in a safe place a few years back and memory aint what it used to be!. I'll machine up a set when I get time.
Regards
Dave