Hi Gerben,
I, too, have an identical box of un-opened Swiss 9 mm ammo that is dated 1943 as well.
I was curious and perplexed about the 9 mm use in 1943; as the Swiss Ordnance did not adopt the 9 mm round until the P49 SIG pistol.
I asked on the Swiss Rifles forum what Swiss Ordnance weapon would have used 9 mm in 1943. "Guisan" indicated Swiss machine guns, did.