40 numbers are less common than 30 ones. Some think these might be manufactured in China.
I have some parts starting with 40-numbers and all originating from US exclusive pirates´ sets, and when compared with exactly the same parts starting with 30 from other sets there is a very slight quality difference, the 30-ones look a little bit nicer/better finished.
So I believe it is very possible that parts starting with number 30 are China-made... Maybe it was once a test or something, to check if it were possible to start a qualitative production-facility overthere?
I remember people talking about other US exclusive sets which also contained several 40-numbers and the sets looked less standard-quality (there were several such vechicle sets, one was a firetruck).
Sofar I also have never been able to order any 40-part from DS Belgium (which passes on orders directly to DS Germany from where they are shipped).