only very early 2800cs seats had the release know at the bottom. this was gone before mid 1970. but those seats had the panel back and the chrome cap trim as well as the tucks / folds at the top ... as well as headrests.
The very earliest fabric seats had no chrome trim strip on the back. Levers down low. Stitching on the seat back with no folds.
I think my notes above about the seats are valid. It came from someone who knows these cars well. What we don’t know is how long BMW sat on those photos before publishing. The levers moved by ‘71 so it seems. I’m not sure when europe introduced the 3.0CS.