Speaking as one who has used microfilm rolls of old newspapers in a public library, I can say that this medium is something that’s pretty much ignored by the public at large. There are so many more appealing and accessible ways of obtaining data, even from yellowed clippings in a manila file folder. The microfilm readers are not fun to use: they might have a glaring light, a noisy fan, dust, etc. The user has to scroll patiently and slowly to look for what he (or she) wants.
All this tells me is that someone went over those rolls with a fine-toothed comb. What was available at the librarian’s desk may have been the real thing—or the real thing may have been plucked out and something else substituted for the curious. This all points to prior research and anticipation on the part of the library staff.
Real or replacement? We probably will never be sure.