My guess would be that he opposition trained their agents to leave space on the first page of their reports for archive file numbers, estimates of source reliab

My guess would be that he opposition trained their agents to leave space on the first page of their reports for archive file numbers, estimates of source reliability, notices about which consumers the information has been shared with, that sort of thing. (This was probably around 1950-1960 or so, so everything on paper, probably often handwritten.)