hammerless and with a round safety button that slides on the left side of stock
interesting gun. a side mounted safety button on a sauer shotgun is VERY unusual. Only the double rifles had this feature, but normally the shotguns not (safety slide on top). Can you post pictures of the left side of the receiver please?
BTW: judging from the SN, manufacturing date is app. 1908. Below SN 200,000 (=1913) estimation of production time is just a guess based on known and confirmed SNs. Only Sns over 200,000 can be determined exactly.
The "70" on the receiver definitely has nothing to do with the chamber length. I liked to see photos of this also.
Next: on the barrel there are the usual proof markings of the time plus the large crown (Sauer's brand of excellence) and plus the "wild man with the club", what was Sauer's trademark.
Said DRGM (what's a petty patent) was for the rib on the hammer axcle indicating cocking (decocking) of the hammer(s).