Traditional forged dropouts used a tab that fit into a slot cut into the stay/blade. This design allowed a single forging to accommodate a wide range of frame sizes and styles. Forging from relatively mild steel meant the dropouts were somewhat malleable, allowing for adjustment and alignment.
Cast, socketed dropouts are less forgiving, and either must come with a range of angles between the stays to accommodate different frame sizes/styles, or use a two-piece system such as Nagasawa:
Investment cast dropouts also tend to be unmalleable, and require much more effort to bring into alignment if/when that becomes necessary.