Peter:

How about a "S" with another "S" slightly above and overstamped w/ a "U" overstamped(Sauer and Sauer or Sauer & Sohn)? Just glancing at the Berlin addresses, it is either father and son or a misspelling(my vote is for father & son). I've seen mostly "O. Geyger" after 1900 and the "O." wasn't spelled out. But the O. Geyger Company in Berlin was the retailer that peddled Sauer's products as well as Oskar Merkel's.
So there's not a date stamp and the gauge designation is in a circle?

Kind Regards,

Raimey
rse