I started on them when I was 10 years old back in the 80's. My details might be a little fuzzy, but they were imported through a company called B & A distributing and retailed through Andy & Bax in Portland, Oregon. The grey/ silver color is the best, most durable material, and yes, it is Hypalon. My first IK was a Riken, and we have had several Riken rafts in our paddling group.
My Dad has a 1996 Riken Pioneer that is still in great condition, despite flips, pins, and regular use. (It was out on the North Santiam this week) It has never had a leak, puncture, or even an issue with the military valves. The only real problem I have ever seen is blown I-beams on the floors. The first raft I rode in was a Riken, and it is still being paddled today. (It's a blue 1986 15' self bailer)
I wouldn't pass up a good deal on a Riken, but I wouldn't pay a fortune for one either. The biggest thing to look out for is moisture in the air chambers. This will rot and degrade the material from the inside out. So if you are looking at one with a patched floor, do your best to check for any moisture in the chamber. The material is only coated on the outside, so water will ruin the backing.
Wishing I was on the river instead of surfing the web...