The cast number, 78 20 501 112 is the part # for the housing. The stamped number, 78 20 900 112 is the part number for the finished assembly. The Fabr. Nr. number 427 is the consecutive build number. It seems casting was changed shortly after to accommodate a build plate. The question I have, maybe Franz knows, is the date on the plated racks in month/year or week/year?

In any case, build #427 seems to be early 64 build. Looks like a very early rack for sure!