There's one on the designer Special cover:

http://www.ninfinger.org/rockets/no...a/69cenp74.html

The X-21 is in the picture

http://www.ninfinger.org/rockets/no...a/69cenp34.html

But the "mystery rocket" at issue is the Centuri Space Shuttle.

http://www.ninfinger.org/rockets/ca...1d/71dcenb.html

The "blackout" was done because the final decals were not done and the mock-up painting completed.

Jerry