You are buying a used vintage Weston Direct Current Millivoltmeter. Model 1. Manufactured by Weston Electrical Instrument Co. Newark, NJ USA. Top post on right marked 2.5?, lower right  (+), top left marked  50 and bottom  left 100. Tested with AAA battery and appears to work properly.  Casing has some scuffs but no cracks. Check pictures for condition.