I know a guy who has an extra Cog. shaft down here...
It's just under 13mm, straight enough for what you do.
It is one of Joey's older shafts from when he first started making cues and actually put some heart into his work.
Call me if you're interested.
-Me
Mike Gulyassy has one regular ring work, brand new 3.8 oz. I have had this shaft for 4 years and it is dead straight. Call me 864-458-7662 or e-mail mike@babysproshop.com