Agreed. If you took a "photo of a photo" in 1930 the resulting product would be another glass plate negative that when developed would render you a second generation (type 3-4) image if using the psa system. I have seen a ton of them. Earlier glass negatives from the 19th century-1920 are generally bigger (around 4X6 inches) and the glass edges are rougher. More modern glass negatives from the 1920's and 1930's are smaller (around 5X4) and machine cut edges.
Rhys
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