OK, just one rivet.  Anyway, here's my situation:

I've never had much luck with the carb on my sweetie's '71 bus
(34PICT3) - always seemed to want to be set way rich, etc....  Well,
we finally got the new motor in and I had the usual difficulty with
the carb.  Further, the timing was jumping around, usually 10 degrees
or so, wildly at low RPM. After checking out all possible vacuum leaks
and swapping distributors to no avail, I finally broke down and stole
the carb (also 34-3) off my bug ('73 'vert, fwiw).  Voila!  Stable as
a rock.  Works great.

Here's where it gets interesting though.  The carb off of my bug (now
in the bus), which works great, has a pop rivet closing off the hole
in the throttle plate.  Doesn't look stock :-) Both carbs have the
hole, but only the carb that works has the rivet.  What gives?  Is
there supposed to be a rivet there and the bus ate it?  Is it a common
fix for some other problem?

Any clues would be gladly accepted...

-Mitch