Back in the '60s and early '70s, everywhere that I worked the drill 
was ridges to the left.  That was  because most people are right 
handed, and you check for the ridges with your thumb.

Bob Cham



>I have a feeling that all these answers have confused some of the
>newbies who have never used a double-quarter inch patchboard -- what Lou
>called a dual pole.  This used a pair of tip-sleeve plugs on one plastic
>base where the two tips were the balanced-circuit conductors and the
>sleeves were ganged together.  There was an edge with ridges on each
>plastic block which told the operator which tip was which.  I always
>taught that the RIDGES GO TO THE RIGHT.  (Both could go to the left but
>it was better to have every operator do it the same way.)  A stereo
>signal required two patch cords -- one for each channel.  So if you have
>one cord with one end reversed, the two channels would be out of
>relative phase.