Mescator, the ship you are referring to is most probably the "Montevideo Maru", see:
http://en.wikipedia.org/wiki/Montevideo_Maru
Sadly, this was by no means the only instance of Allied subs sinking Japanese merchies which were transporting Allied POWs and/or Asian slave labourers (the latter victims almost completely forgotten today). To name but a few:
Kachidoki Maru
Rakuyo Maru
Junyo Maru
Lisbon Maru
None of the hell ships had any distinguishing markings to indicate they were transporting POWs as far as I know.
Tater, no need to apologize- we are all learning from each other as far as i'm concerned
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