Mine is also missing from drivers seat, I tried to fit it when replaced the foam base part but couldn't figure out how it fits at the rear part, as others say doesn't seem to affect any operation of the seat I think it is supposed to assist the height adjustment ie make the lever easier to operate? or something like that. Found this though so might have another go at it sometime...
Remove seat.
Note
Before removal, seat must be moved to highest position to reduce height adjustment spring tension as much as possible.
Remove screw 2 and unclip trim 1.
Unclip two covers 11, unscrew two bolts 12 and remove adjustment lever 13.
Remove screw 17 and unclip trim 14.

Carefully detach tensioning springs 2.
Unscrew three bolts arrows (10 Nm) and pull seat height adjustment element 1 out of seat frame.

Source:
http://elsaweb.spaghetticoder.org/doc/V.en-GB.K05510989/27585173/3