I suppose any overprint which looks badly forged is just, well, badly forged. But could it be a genuine mistake at the printers', or those mysteriously adverse 'wartime conditions'?
Probably not. The one on the right is for comparison.
That green stamp cost me the equivalent of a dollar - I've just ordered another one up at around $1.50. It pays to look closely at the image when ordering online, even if sometimes the image is so poorly uploaded that you can't easily tell what's what.
I only have a few stamps from this set but while there is some variation in the type setting they are all much cleaner than this with more uniform letters.
The diamonds at the bottom of this stamp seem particularly poor.
...hardly that You have fake overprint, as I see in Michel catalog, there are no mentioned forgery or fake overprint. Actually, You have just two value from set of 16 values. Full set of 16 have very good catalog (Michel) value of 150.00 euro in mnh condition, but for sure, I will prefer this set with certificate.
In Michel catalog, it is under number 19-33, from 1941, Italian Occupation of Slovenia:
