Thanks for your comment.
In the "Chess Problems" chapter, the manual says "Due to the computer’s memory capacity, the search depth had to be limited to 32 half-moves".
However, for finding a mate in 16, 31 half-moves would be sufficient.

Later, in the "Seeting the Search Depth" chapter, it says:
Due to the computer’s memory capacity, the search had to be limited to 31 half-moves. Thus, the program has the ability to solve mate problems up to 16 moves.

31 would perfectly fit to the claim that the actual search can be up to 10 half-moves higher than shown on the display: The 21 moves I noted above + (max.) 10 for additional selective search.

Therefore, let's agree on a max search depth of 31 :-)
In any case, I find it interesting how the programmers solved the display of the search depth.

Regards,
Jens