(scan courtesy of Teetan)
Not only it is limited to just 20 pieces but it is also in rose gold (extremely few Mercators were made in RG, they were mainly in yellow gold and platinum). The main rarity is the dial! Mercators existed with an engraved dial representing either the Americas or Africa/Asia/Europe, special pieces were made upon request from specific markets representing a specific map or location, the dials of these watches were in polychrome enamel. For this watch made specially for Thailand (I belive a unique piece with enamel dial was offered to the Thai king) the dial is engraved! To the best of my knowledge this is the only model made for a spacific market with an engraved dial!
A platinum model was also available (20 pieces) with the same dial