Nothing out of the ordinary going on there in the videos
Max travel of the claw arms is aproximately 20 thou measured between claw end & stop bit on the case.
It never reaches that stop point but is for measuring the gap. Gap should be much the same up changing & down changing although there's likely a few thou play in everything moving so can only get close.
The big wobbly screw sets that max travel. Then small wobbly sets centralizing.
"I'd steer clear of all that as looks good & possitive changing".
I see still a fiber & rubber ring still in the basket but is that before total dismantle. all good.
Looking at the steel at a distance it looks like the fibers have left their mark, like had stuck up or close to it while parked long term.
Claw arm wobble in the last video i'd think is normal. Narrow iten has to be less than perfect fit. I don't see the circlip to hold them on but i'm sure you have it there!!.
Plunger & spring in place--sounds like its clicking i possitively.
So i'm no help at all sorry.

This was the second time I took everything apart. I fitted new fibres, removed the o rings (apart from that last one) and took the bike around the block for 20 minutes. Thought it was fixed until right at the end, I could only select 1st and neutral, felt like there wasn’t enough travel in the claw arm to turn the barrel. Somehow by the time I got it drained again and apart, it all seems okay again !!! Not sure if it’s getting too hot and somehow tightening up, head temperature maxed at 120’c not too sure if that’s good or bad or even accurate, got one of those digital plug temperature gauges on the bike.

I’ve just been rereading your comments and I’m presuming that the pin with the nut on it is the an adjuster to allow a little more travel on the gear selector and the stop in the engine is where you get 20 thou measurement. If that’s the case, I’m miles out on that 20 thou, more like 2mm !!  

Hmm, i'd think if you had two mm one end it'd be bumping the stop going the other way.
With the wobbly screw whatever you do one end affects the other.
All the same--I don't really think its anything to do with that.
When you had first & neutral it just 'has' to carry on through to second if you pulled the lever past that point in one movement--as if you were riding it. Unless something is stuck in the way internally.
Got me beat i'm sorry.
I really don't want to say total dismsantle but seems like something unusual is happening-- "right in the middle".
Hope i'm wrong. Check the plunger & spring--can't think why, but just be sure nothing is stuck in & around it but can't imagine what--unless something is broken like a tooth or a split roller on a sellector pin  & getting in the way of selectors moving in their tracks   [but think those pins had done away with the rollers & gone sollid by RT3],or something broken & stuck under the cam plate on the left end of the drum--plunger sits into it.

Looking back in a relevant service manual which in this case is DT1E i doesn't give a measurement but show gap A & A as being equal. Which is full one way & full the other. I'm sure ya all good with what you have.
Maybe i measured one once & got 20thou [was 50+ years ago lol], i'll keep looking but doesn't matter.
I used to do things like grind the arm stop where it stops against the screw on full movement over the screw to get more travel when mine started missing gears.
But that didn't work & started getting false neutrals past the gear ya wanted.
Cause [original cause as it transpired] was caused by rounded dogs throwing it out of gear or not fully engaging , [talking under full throttle & no clutch in a racing mx situation].

attachment=57753]BC2FA114-144E-4759-9FDD-CD050927DC3A.jpeg[/attachment]  
 I’m going to measure the stop end positions now I’ve read the online manual, suspiciously grubby page 
  
Really hoping it’s the offset screw adjuster, it did seem like lifting up the gear lever to get into second just couldn’t quite travel far enough to click the drum round

  

www.yamaha-enduros.com/index.php/service...ntary-service-manual
Here's a relevant pic on page 6 if ya scroll down. All the early motors up to "73 are the same in that respect.

Sorry i put this up before i realised ya found a pic
I edited my other post as they call it A & A measurement --[gap] then B & B which is sorted by A & A anyway. They're just showing that it should stop pulling the drum around before its fully in gear. But is close to the plunger being right home in the cam plate hollow [just short of right in & the plunger spring & gear dogs sort of take over & pull it in the rest of the way.
Good clear pic on my link.

Hate to say it but with the adjustment it's not going to be faulty one minute & ok the next then--get in a mood & play up again. Tis getting like a naughty kid though lol.

I didn’t make a video of turning the drum with a cross head screwdriver because it was too complicated to hold the phone while turning the shaft and selecting the drum but that’s how easy it revolves. I suppose it’s just two stroke fever setting in and I’m getting quite fast at taking it apart and putting it back together   thanks for your time and thoughts I’ll update if I find the issue.

Take the gearchange shaft out of the motor, and check if its bent or not. Replace the cross head screw in the selector drum, with an allen bolt, will make it much easier to turn the drum for testing.  Are there any metal particles in the oil, and have you checked the crankcase for cracks close to the adjuster pins?