That is a vacuum can or reservoir.
The actual vacuum system is unknown. However, the picture looks like it is from a vehicle from the 70s and a full sized Ford or Mercury with a Windsor block engine.
Anyway, the possible vacuum systems are: (in no particular order)
Power door locks
Concealed head lamps - however that reservoir may be larger
Or some other vacuum controlled circuit
The purpose is to maintain a vacuum signal during periods when engine vacuum is low. Like dropping the pedal on the right to the floor. This would cause engine vacuum to drop to near zero.
It's a 1978 LTD. Station Wagon. I was really lucky to pick it up.
One owner from new in the US, the body is in excellent shape just needs paint. Underneath is perfect. It needs a few bits. The trans position selector cable has snapped, and the back of the instrument panel has started to crumble around the instrument panel lights, not a big deal. The headlining is the only thing wrong, it's fallen to bits. Rockauto do a new one for $180 dollars.
Well pleased with the car.
I also have a 1976 Dodge Royal Monaco, 70 thousand miles from new, plus a 93 Buick Roadmaster wagon which I have been working on for a year or so.