Yeah I've seen this in a few places. Hangar 9 sells a rudder pulley - two models actually - one for Futaba and one for JR/Airtronics:
http://hangar-9.com/Products/Default...ProdID=HAN3512

Also this:
http://hangar-9.com/Products/Default...ProdID=HAN3530

I tried putting a JR horn on my Airtronics servo, and it seemed to fit - actually it went on easier than the Airtronics horn. I figure even if it's just slightly different it's no good, so I just took that at face value.

Some things seem to say JR/Air, others don't say anything about Airtronics. I also know that Atx is coming out with new stuff and changed their servo plugs - I don't know enough about them to know if they have one type of servo spline, or many, etc.