Proofmark stamp
I own a FN 1920 32 ACP pistol that is Nazi proofed. It has 2 proof marks at several places. 1 is an eagle over swastika. The other is eagle over Wa and the numeral 140. Wa is for waffen (army) and the number is assigned to an individual inspector. The SS probably denotes an SS inspector. There were SS marked guns of many types. Don't have any idea of value, someone somewhere wants it, you can be sure.