Is this a minting error ?

There are some letters on the face of the coin unknow to me . They look as they are stamped underneath the image. Was it caused during production of the coin or it was made later?

It looks like letters but it's not.  Your coin is damaged.  Something hit and scratched the coin.

 Had to look at the page to see the difference 

N#3416  [not silver coloured for some reason] 

then make an animation > 

  [click to enlarge] 

 ‘There are some letters on the face of the coin unknown to me. 

They look as they are stamped underneath the image’. 

 I made these, trying to see what could be there > 

 So - what else was made around 1939 that is 25mm diameter, 

and has a line of horizontal lettering … 

It didn't leave the German mint like that.  I'm still in the damaged column. 

I think it's unlikely to be random scratches; looks too much like lettering.

Often when a coin is hit with a stamp post-mint, it gives a flattened area on the other side (in this case the obverse), due to the force required to make the impression, and no such feature is visible here.  

So I'm inclined to think it was struck over something else.  

Thank you for helping me . There are certainly scratches on the coin but the horizontal order of the lettering is still strange . Why would someone stamp them on the coin in first place ?

I’m going for PMD