What exactly is the difference between "a 28 series", and "a 228 series"?
Or is that a typo?

I was on oldgmctrucks.com and my carb looks exactly like this:

www.oldgmctrucks.com/photos/AcceleratorControlSchematic.jpg

I will check whether the idle adjustment is horizontal or canted 45 degress, in a couple hours when I'm in the garage, and will post with my update.