Some WaA codes are difficult to trace. There are several lists of WaA codes and probably all are incomplete, sometimes missing any information for some codes, other times giving just partial information. As said above, I think that the inspectors sometimes moved from factory to factory, so in different year, a given WaA number can be found on different products, from different makers. This case seems exactly such. Tracing the movement of the code, we can get the impression of a vehicle technical expert, moving every year or two to another factory, but always vehicle related. Looking through several lists, in 1940 the 691 is found on a 8 cyl. Horch from the Auto Union factory at Siegmar. In 1942 - NSU Werke AG, Neckarsulm. In 1944 - probably at the Steyr factory, for the Steyr 1500 vehicles. And obviously has been at the Mercedes-Benz factory, too.