C11 With Odd Numbering?

I have several top strips of 5 of #C11 that have numbers hand stamped onto them. My first thought is they were used for counting or some inventory purpose. However, the number applied to the front does not match the number applied to the back on any of them.
Is anyone familiar with what the reason is behind these? Or perhaps it has nothing to do with a postal service function and was applied by a collector or dealer?
Any input is greatly appreciated!!

HarryG

I have seen them in the selvage, but never on the back of a stamp. Everything I have read about them assumes that they were an accounting number of some kind.