Yaw modified IDA?
 

Okay, I've got a handle on how these things normally work... but I was poking around the pictures section of Yaw's site (http://www.yawpower.com/picture.html) and found... an IDA modified with a third circut? WTF?

I can't quite tell where the bits on top are going into; I'm not looking at an IDA right now so I dunno if those channels go down to the idle bleed adjuster dealies from where the brass bits are screwed into the top. Or if the brass jet dealies go through into the main bores, which looks more likely given the length of the part turned down on the lathe.

In the first case, maybe a finetune for idle quality somehow (but why?) and in the second, the airflow through the bores acting like a siphon to richen at very high air velocities (but why not smaller air jets?) Anyone have any idea what's going on here?