Very common in Asia. Theft of mail there is widespread, especially coming from or going to the US or Europe. My wife uses it if she has to mail something to Thailand, but even then it can get stolen. Usually she has a friend who is going there hand carry to Thailand, then mail in country to her family. Most thieves ignore intercountry mail, concentrating on international.
Maybe that's exactly what they were doing it for. When I was serving in the US Armed Forces, I had a girlfriend in the Philippines and occasionally would want to send her a little "folding money" for her birthday and such. I had a stack of old 1 step instant pictures of various unimportant things. If you know anything about those, they are thick. I would splice them apart along the very edge with a sharp Xacto blade and put a few bills in them and then glue the small incision back shut. Not one time did she not receive the letters with the pix and money implanted inside. Philippines is no place to really be shipping valuables in or out of. That would be a reason why it was packaged the way it was. Who really knows though.
Interesting story...many 1907-1912 silver Philippines coins were dumped in Manila Bay as McArthur fled the islands to keep the Japanese from getting them. Recovered years later, many of them aren't very good looking coins.
I'm not an expert, but, think that this coin being found in circulation would give some verification of authenticity. I often times found Pilipino dimes in change when I was a kid.
Actually, now after sunset, it does still have a lot of luster in the fields, but high points appear polished.