This vintage 10 volts or 35 amperes battery tester was made by Sterling Mfg Co. in Cleveland, Ohio. It measures 2.25" in diameter. It does work.
You can try with small battery, AA, or C or D. Touch the negative part of the battery with one of the pointy parts at the bottom of the battery tester and touch the positive side of the battery with the tip of the metal part at the end of the cable. If the battery is good, the needle will move to indicate what is the volts in the battery. The tester has two glasses, the top one is broken but the second one is very good. The metal case of the tester needs a good cleaning. It comes with a leather case.
Vintage 10 Volts Battery Tester By Sterling Mfg. Co. Cleveland, Ohio