design changes....when?

[Babybearjs]

enclosed are some photos of my green telephones... take note of the difference in the designation strip holders... one phone is dated from the 1960's and the other from 1972... so, when did they make the switch? the phone from 1972 is the only one in my collection with this style of chassis... was this a fluke, or was this the norm? so, when did this design change really take place....