excuse my ignorance but whats the difference between a S40 and a LS650 ?

Whether you have a Savage or an S40, it's an LS650.  Basically, a couple of years ago Suzuki renamed their cruiser line to use either S, C or M followed by the displacement in cubic inches rather than cubic centimeters.  I've heard various monikers for the letters such as [S]porty, [C]ruiser and [M]uscle but I haven't seen anything definitive.

I believe LS650xx is the model number while S40 and Savage are marketing names but it's irrelevant.  It's the same bike.  However, it has undergone some changes every now and again during its 20-year history.

my s40 is a ls650k5..suzuki still calls them the ls.. i thought mine was an 06 actually mfg   9/05.
other cosmetic changes no back rest, now an option
width and height are narrower and shorter than 04
some torque specs are different..
some other changes over the years: main jet size,fuel tank capacity,5 speed transmission
suzuki put their cruizers under the boulevard  name in
04 (?) C=classic,S= sport ,M= muscle