Analogies between IT and physical world are often less than helpful but before digital photos most people would be developing photos in specialized studios. If

Analogies between IT and physical world are often less than helpful but before digital photos most people would be developing photos in specialized studios. If these studios noticed what they thought was CSAM they would report it to police. One could of course develop photos themselves (like one can selfhost everything today), but as soon as other parties get involved they can (and should) report problematic material to police.