The letters were in the book to mark where the search had been interrupted. The pages there had no entries relevant to the search. But the search never resumed, the letters forgotten, the volume put away. This theory doesn’t claim that the deadly break would have been averted had the locks been changed. The door device fault might have been missed
Inmate Hart hid in the
closet, above, before clubbing Earl.
anyway.
In this scenario, the letters were placed in the book to mark where the research had been interrupted. The pages there had no entries relevant to the search. But the research was never resumed, the letters were forgotten, the volume was put away.

This theory doesn’t claim that the deadly 1948 break would have been averted had the locks been changed in 1944.  The door device fault might have been missed anyway