Longest series of mutual "!" moves Normally I'd pester the/a computer chess forum, but I couldn't register yet...       

On the Matplus chess forum, the following construction task came up.

Both White and Black are on the brink of losing because the position is so sharp. Both players have only one way to draw the game. However, the seqeunce falls under th Nunn convention-each move is the only one that doesn't lose, but it has at least one legal alternative. They are denoted as “!” moves. What is the longest sequence of mutual "!" moves?

Here is an illustrating example:

[FEN "2K5/8/7k/6N1/3q1N2/8/8/8 w - - 0 1"]

1. Nf7+ Kh7 2. Ng5+ Kh6 3. Nf7+


Nalimov confirms that 1.Sf7+! Kh7 2.Sg5+! Kh6! is the only way to draw (by repetition), but since Kh7 is also an only legal move, this position is invalid.

I don't have Nunns "Secrets" series at hand, where surely some examples can be found. Neither can  Imine a database for this question. Maybe you can?
The example below comes from Guy Haworth's Chess Endgame Records.

The annotations, as explained by Guy Haworth, are as follows:

% the only move available
! the only value-preserving move available (i.e., for a White move, the only winning move, and, for a Black move, the only drawing move, all others losing)
''' the only move which does not allow a repetition or otherwise waste time
''  the unique optimal move selected by the defined strategy
' an optimal but not unique optimal move

The other notation used is as follows (this is copied from Guy Haworth's book):

Metrics:
DTM Depth To Mate  

Miscellaneous notation:
AUMS Absolutely Unique Move Sequence

[White "KR"]
[Black "KNP"]
[Result "1-0"]
[Date "2013.11.24"]
[Event "5m, 2-3m, KRKNP, GBR 0103.01, longest-known decisive AUMS: SV+M+/SUm"]
[Annotator "Haworth, Guy"]
[FEN "8/8/7p/7n/k7/8/2K5/3R4 w - - 0 1"]

1. Rd4+! {dtc/m/z = -26/-49/-17m} Ka3'' 2. Kd3! Kb3'' 3. Rd6! Kb4'' 4. Kd4! Nf4'' 5. Rb6+! Ka5'' 6. Rf6! Ne2+'' 7. Kc4! Ng3'' 8. Rg6! Ne4'' 9. Kd4! Nd2'' 10. Rg2! Nf3+'' 11. Kc5! Ka6'' 12. Rg6+! Kb7'' 13. Kd5! h5'' 14. Rh6! h4'' 15. Rf6! Ne1'' 16. Rf1! Nc2'' 17. Rb1+! Ka6'' 18. Rb2! Ne1'' 19. Ke4! h3'' 20. Rb1! Nc2' 21. Kd3! Na3'' 22. Ra1! h2' 23. Rxa3+! {KRKP: dtc/m/z = -3/-15/-3m} {Nalimov and Lomonosov DTM EGTs} *


Each of the first 35 plies, from White's 1st to White's 18th move, is the unique best for the player. What move is best for Black on their 18th turn depends on the criterion applied. Black's 18th move here, 18 ... Ne1, is the unique move which enables Black to maximise the number of successive White turns at which White has only one winning option. However, if Black is to maximise the number of future turns (i.e. maximise DTM), Ne1 is not best for Black: it loses for Black in 20; best is 18 ... Ne3+ (loses in 32) 19 Kc6 (best; wins in 32; Kc5 wins in 34).