H. G. Muller wrote on Sun, May 3, 2009 12:22 PM UTC:
That probably explains it; I don't think that at that time a standard for FRC had emerged.
Anyway, the following system seems a very satisfactory way to handle drops, substitutions and swaps in SAN, as SAN makes distinction between captures and non-captures:
non-capture to empty square (e.g. Kf1): normal move
capture to occupied square (e.g. Kxf1): normal capture, or capture of own pieces
non-capture to occupied square (e.g. Kf1): swap mentioned piece with whatever was on the to-square
drop to empty squuare (e.g. N@e4): drop from holdings
drop to occupied square (e.g. N@e4): substitution, whatever was on drop-square goes back into holdings
warp move (e.g. @@e4): whatever was on drop-square goes into holdings, leaving an empty square
suicide (e.g. @xe4): e4 is made empty, what was on it disappears.