The dime store photo of Robert Johnson is, for everything i've ever seen, is legit. Johnny Shines even recognized the flat top guitar in the photo as being the first one he used. I've not heard of any controversy on the photo in the past, and looked around a bit on the internet today and still can't find anyone disputing it.
As far as the Sears & Roebuck guitars (Silvertone), they extended into the sixties as being the starter guitars (and amps) for the new young players. I know 'cause my folks bought me my first Silvertone when i was ten. I've owned several since (plus amps), and have a few now, including the amp-in-case. They were made by several companies including Harmony and Danelectro.
Truetone was the brand that Western Auto had. I have one. Monkey Ward had plenty of guitars. They were called Airline, made by Valco. Jack White of The White Stripes, has several. He pretty much bought them up i think.
Guitar manufacturers would license out their brand such as Epiphone (and Dwight Argo will surely appreciate this), like the Dwight Department Store out of New York. Epiphone supplied them. Steve Marriott, of Humble Pie, played a Dwight Coronet. I have had two Epiphone Coronets, and have one of them still. Great little guitar with one P90 pickup. It definitely has the same sound as what Steve used.
The lowly Sears Silvertone is living on bigger than ever in the vintage collecting world (considering they were a cheap guitar and amplifier). The Silvertone amps have a great sound, btw.
