Filter, Pneumatic, for Turn & Bank Indicator AN5821-1
This is a used WWII-era Air Filter AN5821-1 as used on vacuum-operated Turn & Bank Indicators such as the A-5, A-8, A-11, and possibly others. It has the female coupling but is missing the male fitting. See spec sheet for the particulars.
If you have one of these Turn & Bank indicators and wish to complete it, this is what you'll need. They are quite difficult to find.