Just a guess...
...and I'm sure John Rogers and others who have been looking into this have a pretty good idea of what might be out there, but it seems likely to me that the great majority of large archives of old photos have been long since consigned to the dumpster, meaning that the relatively few to have survived will produce a comparatively small enough number of truly top quality Type I prints to keep prices high in the future, especially given the increasing demand for them among collectors.
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