You do not come across these every day. This is an interesting and historical guitar amp from the very early days of the electric guitar.
What we have here is a very early, probably circa 1936-1940, tube guitar amp made by Western Electric subsidiary Electrical Research Products, Incorporated (ERPI). ERPI was created as an R&D and service arm of Western Electric. In addition to advances in telephony, Western Electric was responsible for turning Hollywood movies from silent films into "talkies" in the late 1920s using their newly developed technologies, including the tube amplifier and efficient speaker systems.
In the very early days of the electric guitar, indeed of commercial electrical products in general, there were only a small handful of companies making tube amplifiers of any kind. At the time this amp was made, the tube amp itself wasn't that old. Most of the companies that offered amps for sale in their product line did so under license to one of the few large corporations that had actually put in the R&D and made successful tube amplifier designs - companies like RCA and AT&T (Western Electric's parent company).
There is nothing written on the amp, but this was likely one of those which was made under license from AT&T. Many early 1930s amps such as this feature a label on the inside of the cabinets listing this AT&T license, but many others, including this example, do not. There is an outside chance this is one that was made under an RCA license, since it includes a 6L6G tube in its design, which was developed by RCA in 1936, but that would require some more research to determine.
A few components have esoteric codes on them, but nothing I am familiar with. The pot and transformer codes don't seem to follow any pattern I'm used to seeing. The speaker has a "7" date code, which, given the fact that it's an AlNiCo speaker and not a Field Coil, is definitely a replacement, probably from 1959.
The amp's design is simple, but loud, and is surprisingly toneful, with a nice, spongy breakup and a singing, yet raspy timbre that's very unique. (See the demo video above for more on that.)
The tubes are all early original Ken Rad tubes and consist of one Type 80 rectifier, 6L6G, Type 76, and a 6C3. This example has been fully serviced and is very clean both internally and externally, especially for its age. New caps, 3-prong cord, later 1957 AlNiCo speaker, and most resistors have been replaced as well. This means the amp will need nothing for the foreseeable future and is ready to jam. Would make a fantastic studio amp and conversation piece for the collector of very early electric guitar stuff.