One of the first adding machines with full keyboard construction in Germany and Europe was created around 1890 by the engineer Heinrich Proskauer in Berlin. Later the construction of Proskauer was implemented in Sumlock Duolectric 912/C adding machine, which had a supporting electric drive.
The adding machine of Proskauer was similar to Comptometer, the first commercially successful key-driven mechanical calculator, patented in the United States by Dorr Felt. Its numeric keys worked without locking, and the repetition function was not possible.
Biography of Heinrich Proskauer
Unfortunately, little is known about the inventor of this interesting machine—Heinrich Proskauer. He was a German Jew, born on 14 March 1853 in Kreuzburg (Oberschlesien), Kingdom of Prussia (now Kluczbork, a small town in southern Poland). He was the last child (of eight) of Adolph Abraham Proskauer (1803-1875), and Johanna Goldberg-Proskauer (1814-1884). In April 1889 Heinrich Proskauer married in Berlin to his cousin Jenny Unger (1863-1915), and they had three sons (sadly, all of them died in infancy): Siegfried (1892-1892), Martin (1894-1895), and Hans (1897-1898). Heinrich Proskauer died in 1925 in Berlin.