A listing just popped up for a nice working Challenger II, with a 500 CPU and 540 video board (not the later 540B).
Here's the link: https://www.ebay.com/itm/195834401625
To me, this is the classic OSI system. It looks just like my own C2-4P system in its original cassette configuration (the C2-4P had a 502 CPU instead of a 500).
This is also a nice exampe of the gray keyboards used on the early machines.
Dave