Unidentified Instrument

[Gagar]

Newbie here. (Sorry for the previous large images)
My late husband worked for Sperry during WWII and collected several instruments while he worked on bomb sites. I have been able to identify all but one. The attachment does not show everything. Besides the obvious numeral "4", in black font on the left is a "0" of the same size and on the right is "8". In between, are smaller black font hash marks. There are no electrical wires protruding so one would probably conclude it is hydraulic in nature. No other markings are found on the instrument. Thanks for any clues.

 

[Nikademus]

ack! big picture

 

[Night Fighter Nut]

It actually lookes familiar to me but I can't quite place it. I would agree that it looks like it might be hydraulic in nature but perhaps it could also be pneumatic. What does it look like on edge?

 

[T Bolt]

my guess would be vacuum driven.