If that truck has a "Positive" ground, I am thinking late 50's Some forklifts--farm/industrial tractors--cars and trucks--used that set up.
Unless you find a manual at a yard sale or someone who worked on that old of machine, finding literature might be a challenge. I have seen old manuals on web sites like ebay
Wix filter books have section in back that has pictures and a section with measurements to give a part number, take filter to a NAPA store or a WIX filter dealer, they can get one that will work.