These A numbers were written during WW II by the US Airforce on rooftops on a building in city and village in Greenland. They were used as guiding marks for pilots when whey flew from the USA to Europe or their bases in Greenland.
Today only a few are left. There are also an number building in Nuuk, Nanortalik, Kapisillit and Rodebay.
This one is on the building of Narsaq Museum.