The ribs were added to make the teflon bushing less susceptible to issues connected to the wood of the shank swelling and shrinking. When it is dry, the wood shrinks cross grain, causing it to compress the bushing (sluggish action). When humid, the hole grows cross grain, causing clicks. The ribs give the wood room to move.
This was particularly a problem with the earliest instruments, "teflon 1," with smaller holes and bushings. When they discovered the problem, they increased the diameter of the bushings ("teflon 2"), putting them in a larger hole. They didn't communicate any of this, just left it to technicians to figure it out the best they could.
(Current cloth bushings are dipped in a teflon solution, and are known as teflon 3. Well, maybe some still use this terminology.)
I was lucky enough to have a dozen Bs and Ds from 1962 with those earliest bushings, so became rather intimate with their problems until I had replaced all the action parts in them.
Regards,
Fred Sturm
"Since everything is in our heads, we had better not lose them." Coco Chanel