The then current unoverprinted stamps and hand stamps were allowed for postal purposes until the invading force had their system in place. This state of affairs continued until the English pre-war cds were phased out by October/November of 1942. The Train Mail cds were in use for a bit longer.
It is not surprising that these stamps are uncommon. A number of years ago, the full 1939 pictorial set with 1942 postmarks was sold well in excess of catalogue prices. I assume that most of the values of this set would have had philatelic cancels.
This is not strictly a war time stamp being dated 3 Nov 1941. This is just before the outbreak of the war. But there should not be many of these stamps around as most of Europe was already at war and not much mail would have made it there. Whatever were used locally were probably mostly destroyed during occupation or allied bombing.
Reference: The Sarawak Journal