This is..
A very common accessory, just about all tanks and canisters as well as some uprights had a sprayer in the 40s thru the 70s, im not sure just what this one is for , these are used for insecticides, wax, or thinned paint,and yes it does operate on the venturi principle, the machine is set up for blowing, then this is attached to the hose, when you press down on the button it opens the valve under whatever solution you are spraying, these work suprisingly well.