The name marking ("Fr. Ammann") is probably that of a dealer who sold the surplus rifle. Such markings are common on surplus military arms sold off through the normal arms trade (i.e., not war capture).
'Fr. Ammann' is more likely to be a previous owner. Possibly the troopie it was issued to long ago. I believe import stamps must be a country and on the receiver.
The numbers are probably rack/inventory numbers. Peacetime NCO's, everywhere, love stuff like that.
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A marking put on by an owner is not likely to be done using a commercially made stamp; the stamp costs too much to use it for marking a personal possession. That is just too neat to be anything but the marking of a store or a commercial dealer.