A 7.65 P9S with a serial number beginning with 100? You, my friend, may well own the first production 7.65 P9S! The P9 versions all started with 090, and the 600 made in 1973 were given a new serial range starting with 200. Yours fits neatly between these two known Production runs of 7.65 pistols.
Yours also raises the total of known produced 7.65 P9/P9S pistols to 625. Nice!