Ballantine Model 390 NBS A-T voltmeter. Made in 1961. You tell us what it is. Beautifully precision machined out of brass, silver plated inside, chrome plated outside. Micrometer adjusted interior cylinder with fingerstock contacts to the main tube. The "N" connector center pin connects to an insulated disc. The movable inside cylinder has a similar insulated disc on it's end. Inside, the moveable cylinder is a glass envelope with 4 connections. Two connections go to ground and one end goes to the disc and the last goes to the center pin of the "BNC" female connector. Three foot cable with "BNC" male and large lugs included. Micrometer drive goes from 0 to 2" in 0.001" steps. All enclosed in solid mahogany hinged box. Only one available.