This 1981 Ohio Scientific Challenger 8P (missing name plate) is, I believe, the 8P-MF model. It would have been sold with an additional 8" disk drive unit. Click image for larger view.

Here's a view of the controllers that make up the system. Click image for larger view.

This computer does power up and display the garbled video RAM initialization one would expect, but I was unable to progress further without the correct keyboard. Click image for larger view.

When I opened up this machine to inspect it, I found this keyboard inside, unattached. It's not a 542 keyboard, and I am unsure if it is compatible with this system. Pressing the BREAK key does not cause the system to display the monitor menu selection prompt. So that's as far as I have gotten with this guy. Click image for larger view.