Code Practice Oscillator

I found this code practice oscillator on eBay, and purchased it out of curiosity. I haven’t seen one like it before. The only markings are “SER NO 1382” engraved on the hinged cover, and a paper instruction label pasted inside the cover. The type of audio connectors used indicate that it was most likely made in the 1950s. It operates on 4 D-cells, with the oscillator potted in a green resin. The potted module appears to contain a piece of perfboard, and looks like it was cast in an ice cube tray. It is marked “CTI P/N AD183”. I haven’t managed to dig out audio accessories and batteries to test it yet… so many projects! :)

Have you seen an oscillator like this one before? Do you know when and where it might have been made? Do you have something interesting that you would like to trade for it?