At least there is one questionmark which I did manage to lay to rest. Remember these?
These were the Java (this example has an inverted overprint) and Buiten bezit. overprints
of 1908. I wrote about those here.
Thanks to Hans Kremer of the ASNP, I was able to read an article about this issue. And so it transpired that the experiment was held to see whether local postage rates on the main island Java (and Madura which was administratively linked to Java) could be reduced. In short, the idea of the overprints was that incoming letters on Java and Madura with Java overprinted stamps would be counted. This research period ran from 01-08-1908 to 31-01-1909. It never led to a rate reduction and the idea was never brought up again. No results of this research were published at the time. And as far as all those overprint varieties is concerned. well, I suppose for us collectors it's a case of "seen it all before...."!