The classic one-handed backhand grip is where the heel pad and index knuckle of your hitting hand rest on the first bevel of the tennis racket handle (the very “top” of the handle).
At 0:30 in the video above you can see me holding the racket with a classic one-handed backhand grip, and both my heel pad and index knuckle are resting on that top bevel of the tennis racket handle.
A simple way to find the classic one-handed backhand grip is to hold the tennis racket at the throat with your non-hitting hand. Take your hitting hand and put it directly on the top of the handle. Another “hack” you can use to find the classic one-handed backhand grip is to find the western forehand grip and then simply turn your hand and racket over to your backhand side. You’ll have the correct classic one-hander grip.