Your gun was indeed proofed 3.37 = March 1937. As the proofhouse ledger number 540 shows, it was made and proofed in Zella-Mehlis. Neighboring and competing Suhl did not use such ledger numbers.
The WK marks you describe are not those of Wilhelm Kelber, Suhl, as he used the script and seperated letters without an oval. As your gun was made in Zella-Mehlis your WK stamp points to Willy Krausser, an unsung "gunmaker to the trade" there at Hochwaldstr.2. Zella-Mehlis made combination guns usually are not as high regarded as Suhl made ones. Without documentation a "Wilhelm Keitel" connection adds nothing to the value. The USA are full of guns said to be "once owned by Hermann Goering" or other Nazi greats. Any old gun engraved or stamped with the letters HG is ascribed to Goering, despite Goerings guns were never engraved with such mundane letters. He had his coat of arms on his guns. I suspect someone saw the letters WK on your gun and looked for an infamous maybe former owner with those initials.