I think you're right,,It looks like an etched pattern of one type or another and not an engraved one.
Stevens (Savage) used etched 'engraving' on the gussied up 311 made as the Fox BSE and BST models,,some small amt's of scroll and game birds on the frame IIRC.
If the gun(s) were made in the R&D Dep't for testing and show 'n tell to the bosses,,they many times didn't have serial numbers stamped on them even if regular production did.
Missing a date code on the recv'r wouldn't be uncommon either. All sorts of strange configurations got put together in the different factory Research & Pocket Protector Dept's.
That goes for any business I'd guess.
Sometimes those one-of's surface outside the factorys when the shelves are cleared. Serial numbers are sometimes added at that time if necessary. Prior to Dec/68,,they were not on a shotguns.
Just my thoughts.
