The back case of all original Monacos have this engraved upon them to indicate to the watchmaker the tool required to open the case. Because only authorised service centres tended to have them, these tools remain even rarer today than the watches themselves and are somewhat iconic within the Heuer community. Due to this scarcity and the unusual manner of opening, many Monacos have been opened by force over the intervening years, often resulting in cosmetic damage to the case. To open; the watch was rested upon the tool (as seen below). downward pressure was then applied to the plexiglass (via thumbs), the case then "popped" open. Thankfully it is normally possible to open and close the case by hand once the correct method is applied!