I had a '78 OR back in the day, and I now have an '80 CR. I don't know for sure but I think the 390 may have been aimed at the desert arena where they excelled. I think the chamber is the cause of the strange powerband, it is long and narrow. I have a Rocket pipe for it but haven't put it on yet. The Rocket looks more like a regular pipe for an open class bike. They claim it improves the powerband (don't they all). Sometime this summer I plan to bring it back home (it's been in storage since Rita) and see it that is true.
The picture sucks!
Dale