It looks like a tritax but I'm thinking it would have a makers mark, if it hasn't a makers mark then chances are it's from china
It does look like these ones though (wrong colour though I know):
it looks absolutely identical to the levers i have on mine that also have as-521 on them. i bought them from ebay, they were chinese in origin, cost around £17 iirc and were indeed advertised as ball bearing adjusters. they have been great, feel well made and are a good fit.
They are trak tech i have them on my fazer. Good levers. M&P sell them
I don't know who else sells them? Busters maybe?
these maybe ? they do different colours with different col adjusters
I got mine from this place http://shop.the2wheels.com/ they look good and fit well, about £37 inc. postage, from the land of egg fried rice IIRC.