What type of altimeter is this?
It seems to be a C-12 or C-13 sensitive altimeter (SA) from Kollsman Instrument Division / Square D Company, manufactured circa 1940:


(Source)

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)

C.G. Conn Ltd. is a music instruments company (saxophones), but at war time they provided aircraft instruments to the USAF (flying on instruments finally explained!).

Kollsman published handbooks about their instruments. Here is one from 1938. It doesn't cover this model of altimeter, but can be helpful though:


(Source)