Please see Pagoda Notes, Volume 11 No. 1, pages 5 & 6. At serial number 6619 for the 280SL the brake booster size increased from 8 to 9 inches. Before and after pictures are shown. Note that the after picture shows no hook. The bag is just stuffed in as Shvegel indicates. My car is 7749 and does not have the hook. If you will look at the recently posted engine picture of 49er’s 280SL you will see that John’s car does have the hook. His is serial 3820.
Last year at the MB Starfest2018 concours in Alabama, Judge Pierre Hedary dinged my car for not having the hook, unfairly I thought, as I could not recall ever having had a hook. As it turned out I got the first place trophy anyway!
Afterwards, I turned up the information in the first paragraph above. I am convinced that the hook was dropped along with the booster change at serial 6619 explaining why my car does not have one.