Irwin Stambler gives some information on this Lockheed ten-passenger cargo carrier in Orbiting Stations. As described, the vehicle would launch initially from a hot-water-propulsion rocket sled. Two nested fly-back boosters take the payload stage to orbit. I put the image scanned from the book on Flickr, it’s a hair wider than AFMC’s version and shows the artist’s signature. So, not so much a mystery as it is a spacecraft without a name. More here on Aerospace Projects Review, and more again here on Secret Projects.
A Lockheed artist’s impression of a novel method of taking a unique and untried method of orbital delivery and making it even more unique and more untried. As my wife said to me in the giftshop of the Palm Springs Aerial Tramway, “Baby, I’ll take the car and see you up there!”