I rediscovered something useful when browsing my old 1st edition Monster Manual 2 recently.
Beginning on page 139 there is a set of instructions with examples on how to create random encounter tables tailored to your own dungeons. I had never really paid much attention to it all those years ago as I was disappointed with the book and rarely cracked it open. Meanwhile I was busy giving the first Monster Manual and the Fiend Folio a thorough working over. Their covers show the comparative level of use quite clearly. Having rediscovered this gem of a useful tool I have been spending a little time hunting for other previously overlooked things in the 1st edition books.
What if anything have you discovered or rediscovered in the old books?