I don't think your hypothesis is unlikely, though the 746 German proofing date does raise a few questions that others will have to answer.

There recently was a gun for sale in a local-to-me gun store that had a similar history. It was a French-made 12 ga sxs that bore St. Etienne proofs. Then there were Suhl proof stamps and eagles, the date stamp of 741, and the SuS monogram of Sauer und Sohn stamped on both the action and the heel of the buttstock. That one sussed out to be a GI bringback that had either been confiscated (officially or unofficially) or shipped back to Germany as loot and proofed in Germany before being released into the German stream of commerce.

Explaining the Z-M 746, I'll leave to others.

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