If you're interested in the period of the Japanese Occupation of the West Indies or you just like overprinted stamps (Hi, Eric!), this is a great site with lots of information: www.dainippon.nl. The site is bilingual so no need to read Dutch! I've ordered some catalogues from them as I still need to sort my Wilhelmina overprints. Very much looking forward to receiving them. I'm sure I will bore you with more details once I have them!
I understand from the website that after the deluge of various local overprints in Sumatra during 1942 and 1943, things finally settled down a bit in 1944 with just the one overprint in use on the whole of Sumatra. However, things are never as simple as they seem, for not only does this overprint exist machine-printed, but there are also five types of handstamp!
Here we have an example of the machine-printed overprint,
and here's one of the handstamps.
I don't know which one it is, yet, but I will soon, so watch this space!