Not exactly sure what date the ribbed mag took over and the slab side died, but the most common AK-47 (by quantity produced) typically was issued with the ribbed mag; and in my collection, I usually reserve the slabbers for the type 1 and type 2 varaints of the rifle... so basically '56 and beyond is ribbed... Certainly some slabbers were issued to the T3 guns for some time, but me being the purist die hard, I can't stand to see a slabber in a T3....
OK, then you have got ribbed mags being replaced with the bakelite .30 cal mag somewhere in the mid to late 60's...those are tough as nails, and in many respects perform much better than any steel mag produced up until that time. Corrosion resistant, dent proof, self lubricating, lighter, and theoretically cheaper to make. Bakelite continued through most of Soviet 5.45 AK-74 production, until sometime during the Afghan war '79-'89, the plum PA-6 polymer mags were being manufactured in quantity. Then, post soviet mags were developed using another engineering plastic, and this time they were able to make the formula with carbon black.
That's my general late-night synopsis on the mag thing-
-C