I was wrong (again...). The prohibition of 1st category weapons (military)came on 4/18/1939, just before the war!
This rule has been transmogrified along the years but still exists today.
It also looks like, due to the Vienna Protocol, anything made before 12/31/1899 are classified antiques and not subject to regulation, sort of like in the US. Not being a lawyer, I don't know for sure how that works. Could be useful for importing/exporting weapons though.
Best regards,
WC-