The part possibly of a 37-mm anti-tank gun (ATG) sight. Near it was found several cartridge cases for 37-mm ATG, and in this place on the Finnish map was marked a position for ATG. It is possible to see 2 vertical lines with marks 0 and 80 on the glass inside. The marks on the sight - Z.F. 1x110.
If our version is incorrect please write us ( karel-val@narod.ru ) and give us the correct destination of this item.
I hope next photos would help us to guess what is it.