Original instrument made by Smiths Instruments, UK. It is believed to be for factory testing of voltage regulators, is rare and may be unique. There is no part number marked on the device as far as I can see.

This was rescued from a clear-out from the test department at the factory and has been in storage since the late 1970s. 

The instrument has been tested and is working. It is calibrated to give a 10Vdc reading is at the end white band on the read-out. 5Vdc is about a third into the read-out.

It's in good condition (see photos). The inside and outside of the glass would benefit from being cleaned.

The cork calibration seals on the back of the unit were removed for calibration.

Nominal 52mm diameter.

No warranty is offered on this item. Security marked.