I DOUBT THE JAPANESE WOULD REQUIRE OR TRUST A NON JAPANESE LOCAL TO ATTEMPT TO MAKE A JAPANESE KATANA OR OTHER BLADE. AFTER ALL THEY CONSIDERED ALL OTHER BLADES INFERIOR. AND LIKELY THE SKILL WAS NOT THERE.
THEY MAY HAVE HAD LOCAL REPAIRS MADE TO SWORDS THAT WERE DAMAGED I HAVE SEEN QUITE A FEW NATIVE MODIFIED JAPANESE SWORDS OVER THE YEARS. SOME MADE OUT OF BROKEN BLADES AS WELL AS SOME REMOUNTED IN NON JAPANESE FASHON. PERHAPS THESE WOULD FIT AS A ISLAND SWORD.
IF REPAIRS OR REFITTING WAS REQUESTED BY JAPANESE OR THEY WERE PICKED UP BY LOCALS FOR THEIR OWN USE IS UNKNOWN.
LOCAL SMITHS DID MAKE PARANGS, BOLOS ,ECT THAT FOUND THEIR WAY INTO JAPANESE AS WELL AS AMERICAN HANDS. I HAVE SEEN A FEW BUTTERFLY KNIVES WITH THE RED MEATBALL DESIGN IN THE HANDLE AND ASSUME THEY WERE PRODUCED TO SELL TO THE JAPANESE. JAPANESE WRITING IS FOUND ON SOME PHILIPPINE KNIVES AND SWORDS AS WELL (ANOTHER TYPE OF POSSIBLE ISLAND SWORD).
I DON'T REMEMBER ANYTHING SPECIFICALLY BEING REFERRED TO AS A ISLAND SWORD EXCEPT PERHAPS BY SOMEONE WHO DIDN'T KNOW ANYTHING ABOUT THE ITEM EXCEPT THAT HE HAD BEEN TOLD IT WAS BROUGHT BACK FROM SOME ISLANDS DURING WW2.
GOOD LUCK ON YOUR QUEST I AM SURE THE SWORD IN QUESTION COULD TELL AN INTERESTING TALE IF IT COULD SPEAK.
