The islands of Micronesia were part of the German Pacific Colonies before WW I ( part of the chain of the Ost-Asiatische Kolonien—from the Chinese Concessions (Tsingtao) through the Marianas, Carolinas, German New Guinea, and Eastern Samoa.)
Troops of the Ost-Asiatische Schutz-truppe were initially armed with M71 Mausers, then the Gew88 and finally, the Gew98.
The earlier M71s were devolved to the local White German Reservists, and also the local Native Police force under German administrative control.
So supplies of M71 ammunition would have still been around at 1914; Whilst most of the German islands above the equator were occupied by the Japanese in 1914 and retained as “League of Nations” Mandates, after 1919;those below the equator (New Guinea, Bismarck arcipelago, and Samoa, were taken over and administered by Australia, and in (eastern)Samoa’s case, New Zealand.
These two powers stripped the German colonies of all the German rifles, most of which were either dumped in the sea, or “souvenired” back to Australia…most of the Kar71 carbines and Kar88 carbines in Australian collections come from the ex-German Pacific Colonies; one big lot of captured Mauser rifles still at the Brisbane Naval Stores in 1938-39 were burnt and dumped into the concrete foundations of a new cross-river Bridge
(the Storey bridge) quite close by.
The Japanese, on the other hand, tended to utilise “in loco” any captured German equipment for their own administration, and so both the rifles and the ammo could have lasted at least till WW II in some sort of Police service on these Islands.
The cartridge cases could be either initial use in occupying the Islands in the 1880s-90s, or later in the Japanese occupation of 1914, or even afterwards in normal administrative use, or even by civilian Japanese based in the islands; very little is known (in English) about the period of Japanese occupation of the Marianas and Carolinas in the 1919-1942/4 period.
Good to see someone in a distantly related field of scientific research (anthropology) being interested in Modern arms, as distinct from more primitive types, such as arrows and spears etc. After all, anthropologists in 3000 CE will look at the relics of arms of our generations, and wonder how we used them etc.
Good luck in your Doctorate.
regards, and sayonara,
AV Ballistics, Forensic Services