No idea on the hallmark. The handle might be silver, the blade is probably stainless. A close up looks like you have two different metals. Do you have a picture of the complete handle?
Until someone pipes in - I searched Google and found someone with a similar find asking the same question - Does anyone know the hallmark and the reply was 90 indicates the amount of pure silver 90 grams. Also found "90" discussed on Numerical Marks on German French Silverplate...