I found this great little RCA DR 2000 / DR 2010 tester on eBay. For $4.99, it was worth every cent. I cannot find much information on this device other than the markings on the markings on the device itself. “RCA Numitron Digital Display Device Demonstrator 1g1308”
The controls are pretty simple – the knob controls the brightness and power. The switch has three positions. In the center position, the numitron counts through the digits. In the upper position it illuminates all of the segments of the display while in the bottom position, it only illuminates the decimal point on the display.
The power supply itself is odd. I cannot tell what type of cells it uses. The internal mechanism supports 5 cells that seem to be narrower and longer than a C sized cell. To make it work, I wired a 9V battery to the internal terminals. (Oddly, this thing draws so much current that a single 9V battery cannot actually power the device. As the display begins counting, it seems to reset at the 8 position. To get around this, I had to wire three batteries in parallel to support the load.)