I agree with you that SuS could easily just be a process mark on the Steigleder Drilling.

Regards the Roux-action Drilling, in design it closely matches the photobucket pictures (unfortunately out of focus), posted by on this site by Cummins on 11/02/10. The rear sight does have to be erected by hand. The description on page 46 in Norbert Klup's book does seem to match, so yes, it could be a Meffert. The date on it appears to be "11 11", well before the 1929 date Meffert stopped making them. The name on the rib is W. Müchler & Söhne, Neuenrade n/W (who are still in business). They were probably also only the retailer, in the same way another shotgun I have with a Suhl based maker on the rib was actually assembled in Belgium !

I have a photograph of my Roux mechanism, taken because the gunsmith was both impresed by the mechanism and also the amount of rust ! That's since been sorted but these guns are fussy over the diameter of their firing pin springs with anything too wide coming in contact with the adjacent spring on discharge. Getting a hand made replacement for a broken spring and a barrel re-black has taken a year and I'm still waiting.