The Retina Reflex S was Kodak's second Retina-branded SLR but certainly not the last.
This replaced the Retina Reflex and was significant because it offered true lens interchangeability in a fairly compact package.
The Reflex S was a sister camera to the rangefinder Retina IIIS. The two cameras could share lenses.
The Reflex S carried forward the Retina's bottom-mounted film advance. The shutter release remained on the top of the camera but would move to the front of the camera in subsequent models.
Everything else about the camera should seem familiar to most camera users. This camera, like most other German SLRs from that era, didn't have a rapid-return mirror. So when the user pressed the shutter release, the viewfinder stayed dark.
I like the Reflex S, because it's a quiet camera with a decent viewfinder and good ergonomics. Some people have trouble with the bottom-mounted film advance, but if you've used the later folding Retinas, then you'll feel right at home.
And the fact that it can share lenses with the Retina IIIS is a big plus.