Single Comment

Why is this called Fifty-fifty?  Because half the pieces have to be captured before checkmate is permitted.  Probably Dutch the standard form is that the appearance of checkmate and plain check is allowed along the way, and the King just cannot be legally captured til 50% of pieces are gone. There then could be a retroactive question that needs clarification.

Or in a stretch just to be witty, mis-spell it Fifty-five, and then the King is the "fifty-fifth," working backwards from 64 (squares), as though notionally every square has or represents a piece, less the other King's space.

A CV subvariant generalizes Fifty-fifty, to draw out of a hat how many pieces have to be captured before checkmate, any number from 6 to 15.  Then further subvariant of that one is to "philidor" it and make it an odds game, one side has to capture 6 and the other 10 before permitting mate/checkmate.   

The same Breughem made a specific target of 21, http://www.chessvariants.org/winning.dir/blackjack.html.
This CV  like the others of the mid-nineties, Falcon, Centennial, Hostage,  is about 20 years old now.