Wow, thanks for all the discussion of this. A "J" seems a good conclusion for that mark, 1931, as the '58 date seemed a bit "modern" for this gun and market.
Hate to admit it, but given the provenance and apparent target market of this gun, a side plated boxlock makes sense. All those pins reminded me of a sidelock, ergo my initial conclusion: fooled me! Since the screws don't appear to have been turned, I'm not going to take it apart of confirm, however.
And no, no Siemens on the barrel, and expected it as I'd read the prior discussion about acier martele.
Here is a somewhat better photo of the barrel where the NL is a bit more visible, I hope.

If its sling swivels had been all intact, rear one was removed, I'd have shipped this off to Montana for the sharptail and hun hunt coming up. But I do like slings when walking 6-8 miles per day so will baptize this one on local preserve birds to check the choking.