This is a Knight KG-620 vacuum tube voltmeter (VTVM). Along with Heath, Knight sold many different types of test equipment in kit form—a few hours assembly time could save considerable money. For instance, the KG-620 sold in 1962 for $25.95, but it was $35.95 factory assembled.
This particular meter was a kit. Here is a PDF of the manual, assembly instructions, and all of the other paper that came in the box.
A fresh filter cap and a new diode for the power supply.
The meter has two Knight branded vacuum tubes: a 12AU7 and a 6AL5.