The earlier is thicker and has a horizontal marking below the equator. The later lens has vertical striations only.
Early lens is on the right, later on the left:
The newer lens has Hella numbers (not pictured) 6-112129
The older lens has Hella numbers (not pictured) 2-51764 with a prefix K8201
Note the horizontal feature in this closeup:
In this dramatic photo, the older lens is on the bottom and is noticeably thicker than the later lens.
The parts manual lists the main part number in the topic title as "light-diffusing lens, white, for additional headlamp." The accompanying diagram is confusing, however.
Other numbers for the fog light specifically list a part number of 914.631.952.10, "light diffusing lens, white, for H3 + H1 fog headlamp, not for 914 1.8" and
914.631.930.10 has exactly the same description except is a yellow lens.
Paul