Gauge not installed.
You are correct, that space is reserved for an air pressure gauge, but as far as I know Airstream did not actually mount a gauge there until 1988 or so.
The factory installed gauges were (I think) tubed directly to the active air system, so the only electrical hookup would be for the backlight.
There are a couple of other "not used" items on the dash, such as "glow plugs" (or wait to start), and "water in fuel", which are specific to a diesel installation.
It is possible that, if a PO added it, it might be an electrical send unit.
"Suck it up, spend the bucks, do it right the first time."
WBCCI # 1113
Trailer '78 31' Sovereign
Living Large at an Airstream Park on the Largest Lake Totally Contained in Texas
Texas Airstream Harbor, Inc.