Righto, well the news is that I've found a match

And I can see that the splines will only seat onto the freehub body so far, but I don't know to measure that. When you tell me such a large width, this makes sense because of what I mentioned about the 11t cogs being thicker than the 12t ones. But it's still a bit concerning that you don't appear to already have an 11t lock ring- can I just confirm if the cog you measured was 11t and not 12? Just wondering why you'd have an 11t fitted but an incorrect lock ring, since you say you've a lock ring for the 12t. Anyway, I also dug out a few Shimano lock rings and they're fortunately marked 11t so nice and easy with this one