This month's mystery item is an antique battery cell tester (we darkened the words "cell tester" from the gauge on our original pic), and it's older than it looks. Below is an excerpt from Chilton Automobile Trade Journal, 1930, describing this new-fangled tool . . .

The Allen E-51 cell tester consists of a meter in combination with a pair of steel cadmium plated prods, insulated from each other in the large wooden handle. It is a product of the Allen Electric and Equipment Co., Kalamazoo, Mich., and is priced at $6. The resistor strip, or shunt, is made of an alloy having a low temperature coefficient, keeping the discharge rate constant, it is said.