Hungary 1973 Introduction Of Postal Code Michel 2831 Colour Problem

I received these two stamps from another member of our stamp club with the request to find some information about this.
What I see is that on the left we have the normal 1973 Hungarian stamp for the introduction of postal codes, Michel 2831. On the right is the same stamp but with the red colour omitted for 80 - 90%.
A search on the internet did not come up with a similar error. I do not have a specialized catalogue for Hungary.
My questions are:
- Is these variety mentioned somewhere in a specialized catalogue?
- How can an error like this occur during the printing process or has this stamp been manipulated to create this effect.
- Both stamps are cancelled. Does this have some influence on your thoughts?
- any other thoughts you have about this.

Thanks in advance

Not an error but a changeling. You see the reddish background all over. It has faded but not disappeared. The stamp, likely was soaked in hot water, or someone has been experimenting with a solvent.