RE: Clarke and Hild
That Clarke Model 612, based on the Clarke-Gravely label, was made between 1967 and 1978. Looks to be a wet/dry. Should have a float valve to cut off suction when the tank is full of liquid.
Only info I could find on Hild was that the company was founded in 1927, based in Chicago, and sold its assets to Mytee Products Inc. in 2000. The Hild brand no longer exists. Mytee still sells commercial carpet extractors under its own name.