SCHLÜTER & MAACK GMBH.
image by Klaus-Michael Schneider, 31 March 2007
The white flag has a central shield with a yellow field, a red-clad arm and a white hand holding a blue key. Schneider explains Schlüter as the person who locks a door and holds the key. In the Meyer source the letters are smaller and of even width, unlike the drawn version.
The Lloyds 1912 scan shows the arm extending to the shoulder with a padded-clothing effect. Rosanoski considers the supposed key to look more like a jemmy.
The Massary 1930 source also supports lettering of constant width.