Well they must have quit putting air bleeds in the middle of the base plate a long time ago cause on my 600 holley I have and two others I have cam across there was none.
I also got a spare 600 a couple of months ago and it has the same base plate, tower, metering blocks as the holley 670 street avenger and none of them has an air bleed on the base plate at the bottom.
I don't like the fact of it being there as it could vibrate loose and fall out into the motor. I am going to just take mine out and leave it. I am not running a big cam or anything and my other holleys ran fine. It seems the hole is the same size as my other base plates.
Also I never knew the base plate has any affect on idle mixture from reading all of my holley books from that center mark.