This all has to do with the evolution of the table lamp!
Back in the 60's, two prong outlets actually had no size variance between the hot and the
neutral. Few of these are left. Once the change was made to having the
neutral blade larger, the next logical step towards
safety was to try to get the
neutral routed to the larger surface area of the screw
socket. This is when "ribbed"
zip cord started showing up. As long as I can remember, the rib has been on the
neutral and has been connected to the threaded (easier to contact with finger) part of the lamp
socket, and to the larger blade of the two prong
plug.
I would suspect if the
NEC was a person, the old
Edison medium screw
base on the table lamp would be the bane of its existence, causing much rage!
Talk about something that violates every
safety feature you can think of! But there it is, "
grandfathered", so to speak. Still, at least we can make it a little safe......