Don't know exactly, but we could start as follows. The Sauer name probably has Brand Value level four (BV4 - lesser known or valued Continental names) as I don't think it would have value as would BV3 names like Lebeau, Bury, Francotte, Brancquart,or Masuqelier. The gun sounds like an Original Quality grade five (OQ5 - 1/3 engraved SLNE). Current Condition level sounds like "significant use" which would be level three(CC3). BV4-OQ5-CC3 = $1575.
The gun sounds very typical of the general purpose "duck gun" of this time frame. Clearly neither a game gun (think upland) nor a heavy wildfowler. I'd recommend light loads at low velocity and low pressure just to honor the old wood.
