Depending on the maker, the RBNr. codes first saw use in 1942. Some are dated, most however like this example are not. @luftpirate is of course correct on the maker. W. K. Stolla Söhne was also assigned the three letter code of cgu and I have examples confirmed from them marked Stolla-Wien > 1942, cgu42 and cgu43. Different marking styles were clearly used by some makers during the same year, making the exact date of manufacture difficult to determine in some cases.