Smelling salts are compounds of ammonia used to revive someone who has fainted or is feeling faint. These compounds have been used since Roman times and Smelling salts release ammonia (NH3) gas, which triggers an inhalation reflex (that is, causes the muscles that control breathing to work faster) by irritating the mucous membranes of the nose and lungs. The action of breathing in deeply reverses the fainting and revives the patient.
Smelling salts were standard items in both civilian and military first aid kits of the Second World War, frequently referred to as ‘aromatic ammonia’. The ammonia compounds were sealed in small glass ampoules that were then wrapped in cotton wool and tied into a secure package. To use them the ampoule was crushed and the gas released, the glass being safely contained within the cotton wool.
These ampoules were obviously fragile, so they were held in small metal tins to protect them and it is a small tin that would have contained twelve ampoules that we are considering tonight:

A paper label is pasted to the top of the tin, and although a little rusted and damaged now, the lettering indicating the contents of the tin is still clearly visible:

The design of this label corresponds with that used on British Army examples I have found online, so I am happy that this is most likely a tin used by the military. The lid of the tin is hinged across the back and the tin can be opened up to access the aromatic ammonia ampoules within (now long gone):

An American soldier describes their use on him:
I had one used on me once.. Legit! I was at sick call in basic training. I had a bad infection on my hand and it needed lanced. Well.. It wouldn’t drain so the nurse kept digging around. It hurt so bad, I fainted. They used the salts to wake me up before I fell off the exam table. Worst. Smell. Ever