If it will help the discussion, I can assure you that "Hotchkiss 2-pounder mountain guns" built either by Hotchkiss or the US licensee, American Ordnance, which were made in or after 1899, regardless of ignition system, are NFA weapons requiring Fed. registration.
I am painfully aware of this because of a registered 1899 example with very rare US Army markings that lived about a mile from me, privately owned by a friend. He wanted to sell it, and I wanted to buy it, so I we did all the transfer paperwork and sent it to the local sheriff as required, on its way to BATFE. The sheriff refused to approve the transfer paperwork (or anyone else's during his short term in office,) so the friend wound up selling the gun to some out-of-state "class 3" firearms dealer. I am not going to discuss it further as this board frowns on that kind of thing, but what I've written so far I thought would be helpful to the poster.