C6 GT 390 carb's bowl vent:
IMG_0337 by
Drew Pojedinec, on Flickr
Here are some pictures from Bill's Bk/BJ's
One distinguishing feature is that aside from the regular common, 4160 transfer tube, there is also a "balance" tube which connects the primary and secondary bowls at or below float level.
IMG_0348 by
Drew Pojedinec, on Flickr
Original floats I've seen on all original 3x2 carbs and all 63-64 BK/BJ carbs
The static setting for these is to line the solder up with the bottom of the sight hole, NOT level with the roof.
IMG_0349 by
Drew Pojedinec, on Flickr
All BK/BJ's I've seen have these bowls.
Three things to note:
-Full time, open vent
-"False" accelerator pump thingy
-Cast circle in the front of the bowl..... What is that for? Well some late 50's Holleys actually had a float adjuster that could be accessed if you took a plug out from that location.
IMG_0367 by
Drew Pojedinec, on Flickr
Shot of the primary and secondary vents, front is just a hole with the sheet metal cap, the secondary is a pressed in brass piece. I actually make these pieces (not for sale), for when they are missing on originals.
IMG_0384 by
Drew Pojedinec, on Flickr
External vent on a -U carb, many early 1850's and lotsa other early carbs have these as well.
IMG_0626 by
Drew Pojedinec, on Flickr
*disclaimer*
This comment is in no way related to Doug's carbs, as the dates on his are far later than anything I've seen.