I’ve managed to dig up a photo of the interior of the Potomac Diner, formerly of Hagerstown, Maryland, which answers the question, “factory or home-built?”. Strangely remodeled Brill with a dining room addition off the side? Re-purposed trolley? Two-by-fours and some nails? The pictures lead me to believe it was definitely an on site job, and a pretty quirky one at that.
Yes, it has a barrel roof, so it’s going for that “true” diner look, but look at the way it’s executed. Shallow, and with huge beams on the inside of the ceiling to give the curve. Voila- barrel roof on the cheap that you could build in your back yard. The stools have skinny, tall bases and thin, flat tops like you would see on a ’20s stool, but with later style tube backs. The Waffle Shop in Washington DC had similar stool tops. The counter also looks like an early piece. The diner opened in 1942, so maybe that counter was a salvage/second hand piece from somewhere else? The entire thing reminds me of early home-built lunch wagons, but on a much larger scale.