Let's talk about what kind of things a DM might have in their DM tool chest to do their DM thang as effectively and efficiently as they can.
For starters, they need the rules/guidelines for the game. In the case of AD&D 1E, I would say that should be a minimum of a Players Handbook and a DM Guide. The Monster Manuals are a great supplement. The other books, Fiend Folio, Deities & Demi-gods, Unearthed Arcana and others are interesting but, in my mind, not necessary at all.
The next thing a DM needs is paper. Lots and lots of paper. Lined paper, blank paper, graph paper. There are so many things that need to be written down, kept track of, drawn out and such that paper is pretty much a critical need.
Lot's of pencils come in handy to go with all that paper. In all truth, I find mechanical pencils to be the most handy. Match that to a good, separate eraser and you've got a good start.
Related to the paper issue is post-it notes. This is a great way to communicate things to individual players when the rest of the party can't or shouldn't know what that particular player needs to know.
Obviously, having all the dice is critical. Dice are not cheap but you can't have enough of them. The ones I use the most are 1d4, *d6, 1d8, 2d10, 1d12 (not really used all that often but really handy when the situation does come up for it), and 1d20.
Note the d6 is special. It's my opinion, and many other DM's I talk with, that you just can't have enough d6. I like to accumulate multiple sets of dice to have handy. I encourage players to go get their own set of dice but not all of them , especially new players, know they should have them. So, I keep about 3 or 4 extra sets of dice handy.
A screen or place to roll dice secretly is , in my opinion, very important. I have heard a couple people comment that they do all their rolls in the open. In my games, I like to implement monsters and situations that bring some surprise to the table. In some cases, I let the players roll for the surprise and other related situations like initiative and saving trows. In other situations though, the PC's can't roll for a save or surprise if they can't see it coming. in this case, you need a device to roll those behind the scenes.