the consensus is that it is made by Umarex even though it is stamped Walther (Umarex owns Walther). There are proof marks on the slide that I believe are Umarex.
Correct. This pistol is not
made by Walther at the Walther factory in Ulm, Germany (where pistols such as the P99, PPQ, and PPS are made). It is made by Umarex at the Umarex factory in Arnsberg, Germany (outside Cologne). The proof marks bear this out.
Umarex has taken to stamping "Carl Walther" on a lot of their pistols, because, frankly, "Walther" sounds a lot better than "Umarex" (who is better known for airsoft toys and starting pistols).