There are two canisters that are on the 71-3 Mustangs, a vacuum canister for A/C equipped cars and a charcaol vapor canister that is on all cars.
The charcoal canister is mounted on the fender apron, below the P/S hood hinge, is painted blue and has one small hose connection and one larger tube connection. It is filled with activated charcoal. The purpose of this canister is to catch the fuel vapors from the gas tank and hold them until the next time the car is started. The small hose connection goes to the steel line on the firewall, which runs to the vapor separator on the tank. The large connection is attached to a paper/foil hose that is connected to the air filter housing. There is a plastic vent cap in the middle of the canister, which allows fresh air to enter when the engine starts, purging the vapors from the canister.
The other canister is attached to the rear of the P/S shock tower. This is the A/C vacuum reservoir, whose purpose is to hold a reserve of vacuum to keep the operation of the system constant. The A/C system is vacuum powered and if vacuum suddenly dropped, it would allow the vacuum motors to relax and change the settings of the system. There are two connections on this canister, a medium size that is connected to manifold vacuum, typically at a vacuum tree ont he firewall, and a much smaller nipple that has a hose that goes through the firewall to the A/C selector switch in the dash. There is another small hose of similar size coming through the firewall, this hooks to the flow control valve on the heater hose.
That being said, if your vacuum canister holds vacuum, then don't worry about the rust rattling around inside. Clean it up, repaint it and return it to service.