[RichardJ]

AVS actually stands for "Air Valve Secondary". I've had a Carter AVS on my 292 for about 18 years now.
My throttle cable loops around and over the valve cover. My original, but modified oil bath was even farther modified after this picture to contain a paper filter. I tried, but never could get it to stop leaking oil all over itself.

I looked at the 4bbl carb orientation from the viewpoint of intake manifold runner length. Generally long runner lengths are better for low rpm and short runners work better for high rpm engine power. An inline engine with a single center mount presents a mix of both and a slight imbalance. Inline engines with a single center mount tend to run lean in the end cylinders and richer in the center cylinders.

If you use a string or narrow tape measure to check the distance from the head flange to the carb primary valve it is very long for cyl 1,2 as well as cyl 5,6. Cyl 3,4 is very, very short, especially if both primaries are inboard.
On my Clifford, the inboard ports are only about 4" from the 3,4 head flange, but the outboard ports are 6". The forward primary port is 11" from the 1.2 flange and the rear primary port is the same 11" to the 5,6 flange.

6" vs 11" isn't ideal, but a lot better that 4" vs 11" if both primary throttles are inboard.
If the carb is turned 90* to the head, one primary is 4", the other 6" from the 3,4 flange. Two primaries are 10" from the 1.2 flange and 12" from the 5,6 flange.

Truth be told, it doesn't seem to make a bit of difference, especially from the seat of the pants of most drivers with that setup.
Personally, the thought of that intentional imbalance drives me crazy.

The real world advantage of the sideways carb is the original style throttle rod linkage.

You might think that at WOT, all ports are equal distance in any orientation, but most of these carbs have smaller primary bores. Only the Carter AVS used on the 440 had 1 11/16" x 1 11/16" pri/sec bore. The Ed 800cfm AVS2 has even larger 1.75" x 1.75" pri/sec. All the other AVS carbs use smaller primaries.