I have seen several different builds of the Sears 5XL amp on e Bay and elsewhere on the web.
But the original manufacturer of the Sears 5XL is hard to nail down.
At that level it’s easy for the player to control the output and vary the tone from clean to full distortion. It sounds really good with a decent analog delay or analog reverb pedal. Hope you have a good set of speakers connected to your computer.
The stamped 7-digit code on the pot indicates it was manufactured by CTS (code 137) on the 48th week of 1968. Even though this little amp is as basic as it gets, it simply excels at what it does. But fare warning: manufacturing quality of this amp was inconsistent and not all samples found today will be built like or sound like this one.
But I suspect that even later models were built for Sears by others, maybe using leftover Danelectro parts?
UPDATE: I recently noticed that the schematic diagram for the Harmony H303A is nearly identical to the Silvertone 1420 and Sears 5XL.
UPDATE 2: This basic amplifier design was used throughout the radio and musical instrument industries for decades.
Which makes it even harder to pin point the factory of origin.