These are refill propellant cans for the 5 oz models of Falcon's Sonic Blast Signal Horns.

The propellant is the new HMO1234ze non-flammable propellant that has an ultra-low global warming potential (GWP). It has a 99.5% lower GWP than HFC-134a and is ideal for uses where flammability is a concern, including boating, manufacturing plants, construction sites, etc.
It is not considered HAZMAT, and needs no special documentation.