Most of the 20th century bikes were made with "horizontal" (actually downsloping a little) rear dropouts and the wheel axle was set in the slot by eyeballing the tire between the chainstays for centering. A few dropouts, usually higher end ones, had little, fragile screws that were used to locate the axle in the fork
with instant indexing so you didn't have to eyeball the tire for center placement. Wheel was pulled back til both sides of the axle butted against the screws. Vertical dropouts are now universal and they index the wheel axle automatically in the top of the slot everytime. Steve