As a complement to the "indicator-frame" here is the references of the cited book : The inventor is Dr Julius Paschke The book : "Der Neue Baurahmenimker" by Maria E. Paschke (1952) French edition : "Le cadre-temoin", J.Mosbeux, Bruxelles, ed. (1973) I don't know if an english edition was published. Regards. Jean-Marie.