Why did Vikings have 'Allah' on clothes?

That is interesting PO1 Tony Holland. Since the Norsemen were adventurous seamen who traveled as far and wide as they could. it is not surprising that some may have traveled around the Iberian Peninsula into the Mediterranean and reached North Africa most likely but possibly Asia minor.
The Norsemen were of mixed faiths and it seems they adopted faiths of where they occupied from the British Isles [Christian and druid] to it seems north Africa [Muslim].
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Could it be as simple as trade goods or plunder?

A good bit of written history that we have about the Vikings came from the Muslim traveler Iban Fadlan. So it would make sense that there were other Muslin interactions, especially in trade. There have been findings at Viking sites where jewelry possibly traded between Scandinavian traders and those of Muslim areas. Not long ago a woman was discovered wearing a ring with Allah inscribed on it.