Is this DIN connector suited for MIDI devices?
While one might expect that the socket's contacts would be a round metal receptacle, there is another design that looks like a fork. The two extensions of the fork make contact with the plug's pins by contacting the pin on either side.  This fork-contact has a flat profile, and during manufacture, it is inserted into the slot that you see as a "line".  The sides of the fork also flex inside this slot in order to provide a spring-loaded contact with the pin from the plug.

So these "lines" (slots), and the angle they are made at, have nothing to do with the pin arrangement in the socket.  Only the round hole where the pin actually enters is a factor in whether the "pins are in the right place".