Sanborn Real Photo Postcards--Red Rocks, Colorado

I'm working on a collection of real photo postcards of Red Rocks from when the amphitheatre was being built. I can't seem to find any information about how many images might be in this series to know when to quit and have them professionally framed as a collage. Number appears after the photographers mark, is there a listing of the corresponding numbers that might offer any clues?

I'd be grateful for any assistance, thank you
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