Perhaps I should have expressed that differently: The Model 18 was of generally similar outward form to the basic Model 8 but was built to a higher standard and a somewhat different spec. But I don't think the Masterpieces were necessarily just outwork as the book JP Sauer & Sohn by Peter Arfmann and Rolf Kallmeyer (Peter Arfmann Verlag 2004) states of the Model 18:

"The best specialists at the factory worked on these masterpieces and sometimes also at home. Action maker and master lock maker Paul Werner was one of these. He worked with two assistants in a periferal area of the factory. In the workshop they set up there, far from the hustle and bustle, these men produced the finest locks".