It seems to be a C-12 or C-13 sensitive altimeter (SA) from Kollsman Instrument Division / Square D Company, manufactured circa 1940:
The only difference seems to be the marking "ALTITUDE" replaced by "SA-91". As this is the 39th in the series, this could be the initial marking, later replaced by the more standard one. I don't see what could be "91" for.
The Kollsman window is black (as commented) because the setting dial has been turned past maximum pressure reference, as indicated by the bug between 7 and 8. The window value and this bug are linked.
The brown color seem to be related to phosphorescent markings and needles.
The C-12 has apparently many variants, likely designed on demand, especially for the USAF, like this one with alternate colors for the marks:
C-12 manufactured by C.G. Conn Ltd., (source)
Kollsman published handbooks about their instruments. Here is one from 1938. It doesn't cover this model of altimeter, but can be helpful though:
Of interest is the serial number on the face of the instrument. Yes it appears to be the Kollsman type but lacks the barometric readout in the "Kollsman window".
I suspect that you have an early prototype of the Kollsman. At least that is my best guess. I'm not an expert on antique altimeters but just a knowledgeable pilot and aviation enthusiast.