Unusual Dog Tags

Hi all:

I live in the Philippines and I recently acquired a group of dog tags found buried in the Manila area near New Bilibid Prison (a former Japanese POW camp). The strange thing is that these tags don't have names, just numbers. One many, the common theme is that they all start with the prefix 5IJ and the numbers are centered in the middle of the tag.

On others, there are only a few numbers on the top left.

Any clue as to their origin? I've attached some sample pics.

Thanks, Bill