If you are collector of fine Rolex watches this is the ultimate Explorer. The watch was originally sold for around a couple hundred dollars in the early to mid 1960s. The dial on this watch is a guild dial with railroad track minute ring, and exclamation point under the 6 oclock marker.
Inspired by the Mercury astronauts' first visit to Japan in 1963, the watch was only produced as a test. If the watch was a commercial success, Rolex would then go ahead and re-brand the Explorer as the Space-Dweller for the Japanese market. Needless to say, it was not a success and very few of the Space-Dwellers ever made into the open market.
The Explorer was chosen to commemorate the astronauts' visit because they were (at the time) considered the world's greatest explorers.
Here is a video of one of these rare Rolex Space Dweller 1016.