Hi, got the canisters off and slide the air forks out, pulled off the boot and theres a threaded ring holding the seal in. It has 4 cutaways that appear to be for a special tool or just the right size screwdriver and a hammer.I'm having trouble undoing this. All the manuals and diagrams etc say there's a snap ring there. But not these forks. Any suggestions or should I just beat it more.
Tanx    

What you're describing sounds like what was done with much later forks. 

I looked at parts diagrams, manuals, and pics from threads of rebuilding the 1976 air forks and every single one has a snap ring holding seal into outer tube.  
Pics might help someone ID what you have.  You need to use a 3rd party photo host like Imgur and post link here.

Ahh, I think your right again. I guess they could have modified later forks to take the air canisters. They do have the ridges down the out tubes and are 36mm. So everything is X apart from the seal fixing arrangement. What later Yams had Air forks?
I've tried various ways of added a photo and have failed so far.
photos.google.com/photo/AF1QipNoltGetjcg...r-8FFB3dj9H7NOAOopFY

Ah, on reading the Clymer manual again "On models with a ring nut which holds both fork tubes together, the oil seal is located inside the ring nut" But the "Figure 21" doesn't really relate to my air forks. Aggh.

From. What I remember & I had the 250 in "76 it was one year only on 125 250 400. Those 3 were straight axle forks. I think 77 D was the first with leading axle on all 3 models. Not sure why someone would put air canister tops on later forks. I was happy to get rid if mine & put springs in. Are your forks straight axle?. I see ya using google photos & I am too. When you have your photo securely on your google photo site just click on it the up top right there's 3 sided triangle, just tap that & it brings up a page to create a link code. You tap something lower left to create the code. Then copy the code & paste it here. Only comes up as a link but that's ok. See how you go. Hope that's how I do it. It's sort of become automatic for me now.

Hi RT, this link photos.app.goo.gl/mhkxH3mf9RW7Fpby7

My forks are straight axle.

DONE, got the ring out and the seal. I looked at some Yz125x 1976 forks for sale on US ebay and they all have this threaded steel ring.
photos.app.goo.gl/EMWxPMsQVoAuP8Wk8

Well done & I'm learning all the time even thought I know everything haha , yeah right. I don't think I'd be brave enough to dig the seal out without pulling the fork stantion out first if you did it that way?. Allen bolt underneath let's the station come out. Obviously I never changed seals in mine in "76 if 250 the same.

Interesting.  I had a racing buddy back then with a 250 and I remember a snap ring.  He eventually installed conventional springs and got rid of the canisters. 

Memory could be bad though about the snap ring though...  and I never worked on a set of the 125/175 air forks.  That nut design is very similar to modern KYB forks!

Thanks for posting!!!!  

Yep, can't get at the oil seal. There's another thinner ring under the threaded one and I'm going work on getting that out today.
Can't get the stantion out without removing this and the seal. Undid the allen bolt at the bottom. Tried to do it back up and it just keeps turning. SO, now I have to pull the whole show apart. Could do with a good clean anyway.
photos.app.goo.gl/22WNJcGni9hwK48B8
Cheers gents.