Most probably the least expensive Explorer model there is, the 5506 is an oddity in the World of Rolex, gold plated case, odd lugs, based on a Air-King model, this is a rare watch with perfect dial from an era when a lot of dials get damaged and often need to be replaced. The watch has a 1530 movement, predecessor of the 1560 and 1570. It performs well.
 
Details:
 
Brand: Rolex  
Model: Explorer
Reference: 5506
Year: 1964
Material: Gold Plated 
Dimensions:  34mm
Crystal: Plexiglass Crystal
Movement: Automatic
Bracelet/Strap: Wristwatchme Leather Band
Box/Papers: No/No