Numbers On The Back Of Stamps From The Netherlands

I am busy sorting my Netherlands stamp collection and have noticed that there are a number of issues - predominantly definitive issues but a number of commemorative issues too that are listed with a number on the back like the image shown below (not mine).
As I have searched countless examples and haven't found any such numbers on any of the stamps that I have,
My questions are:
What are these numbers?
Are they evident on used stamps or do they wash off after soaking?

Hello,

These numbers were/are used on coil stamps. Every 5th stamp recieved a number, This to make it easier to take stock.

The numbers of older issues come of easier then the modern ones when soaked.

It can be a sciecne to figure out what type it is. Left facing right facing number, type of number used, etc etc.

Moose, this is the same as on some other countries, like for instance the US!

Peter

Thanks for the replies guys, much appreciated

Hello Moose

The best answer I can give you regarding soaking id from my own personal experience. All I can say is, some will soak ok leaving the control number and others do not. If you have a stamp on piece you want to soak, there is no way of knowing if there is a number on the back or not. To sum up, it is a game of chance :(
Hope this helps?

Chimo

Bujutsu