These images are too small. Please aim at something close to 600 x 600 pixels in size, enough so the mark is readable. It should say more than just "21."
As far as I know, WMF was using it's initials as its mark from its beginning, in 1880, up to about 1900. That age jibes with the spoon's style and the fact that this pattern does not appear in the 1906 catalog nor any subsequent catalog that I'm aware of.
The number stamped on the back of a handle of a WMF place spoon usually means the number of grams of silver used to plate a "standard plate lot." Most WMF spoons this size are stamped "18," so "23" would be a better-than-average thickness, which is probably part of the reason it appears not to have had any plate loss, despite its age.