As regards the "Likeness" of the two ZW meters: The "New ZW" meter has a button that is used to "Calibrate" the ZW unit, the meter covers this hole on the ZW, so an extension is provided in the meter to access this button on the "New ZW"; the "Old ZW" meter has a hole where the button would be, but there is no button, as it is not needed. I do not know if there is any other Electrical differences between the units based on the electronics of the two types of ZW's. Hope this helps. Dennis M.
