From top to bottom, Venera 9, 10, 13A, 13B, 14A and 14B.
Venera 13 and 14 have some kind of teeth on the edge of the lander. What purpose do these serve? Why didn't Venera 9 and 10 have them?
The teeth served an aerodynamic function.
...metal teeth were added to the periphery of the impact ring in an effort to reduce the spin and oscillation during the descent and prevent the rough landings experienced by the 1978 missions.
This is also why the earlier missions didn't have them, they were added in an attempt to mitigate problems experienced on the earlier missions.
Huntress & Marov, Soviet Robots in the Solar System, p. 322 (selections, including this page, are available at Google Books here)
I think the teeth should be used for camera calibration. The very dense atmosphere of Venus outside the camera window of the lander had a magnifing effect like a lens.
This effect is known from scuba diving masks, everything looks closer and bigger because of the water outside is more dense than the air inside the mask.
The exact magnification was unknown before because the optical properties of the venusian atmosphere were not known precisely.