The answers have been pretty much given but ....... an approx chronology .......
IIRC the first R9 design was sightless and then the decision was made to make slides with sights. There was therefore in early days a choice of R9 or R9s ..... my first one was R9s but as was said .... ATF had a problem with this and ''s'' was dropped.
Due to very small uptake on sightless models a while back it was decided to make only ones with sights but - a special order could quite easily be fullfilled for sightles by an extra machining operation on a now standard sighted slide.
You'll see the ''s'' here on my first one but not much later that had been dropped. Blue CF grips of course on both at that time. Opps url wrong first time!