I bought 4 dozen of D'Andrea guitar plectrums very difficult to find in Italy but I found them of a strange shape, form of an arch and I do not feel at ease, I ask you if it is normal that they are of this form or is it a factory defect? cordially

This is not a factory defect.

difficult to tell without knowing the model and what yours look like. Go to google image search and search for your exact model... then you'll know. But honestly if you got a weird shaped one keep it.. those are rare and collectible.. and they probably still play real good. Or maybe the shape is special to give you a specific sound. Either way, good luck!