They only started doing it in 2005.
The C50 and M50 are fuel injected and the S50 is a very cold blooded carburetted.
The C50 and M50 weigh about the same (544 lbs dry) and the S40 is about 100lbs lighter.
The M50 and C50 have a 4.1 gal fuel tank and the S50 has a 3.2 gal tank.
All these things lead to the M50 and C50 being better suited to longer distance riding - for comfort, stability and better range.
The S50 however is the fastest and most nimble of the Zuki 50s. It will hold it's own in acceleration against many bigger displacement bikes. Here's a short list..
HD Sportster 1200
Kawasaki Vulcan 1500 Mean Streak
Suzuki Boulevard S83
Honda VT1100 Shadow Spirit
BMW R1200C
Yamaha Road Star 1600
Yamaha VStar 1100
Suzuki Boulevard C90
I'm pretty sure that the only lower displacement cruiser that will outrun the S50 is the Honda Magna 750, but that's a V4.
The S50 is a great day trip canyon carver. It can feel cramped after a bout 6 hours in the saddle. The longest day I've had so far is 11 hours in the saddle. I'm not sure I'd want to go any more than that without an overnight rest.