Strange… it looks like a manifold pressure instrument rather than a boost gauge. If it read in inches of mercury, it would indicate almost exactly standard sea level pressure (29.92″) but the instrument face shows psi, which would equate to 14.7 lb at sea level. Then there’s the minus scale to consider…
It could well be a Negretti & Zambra calibration instrument of some sort, as suggested. Boost capsule testing, maybe.