Page "M18 Claymore mine" Paragraph 31

The sighting for the device was also originally intended to be a cheap pentaprism device, that would allow the user to look down from above and see the sight picture.

After finding a suitably low-cost device, it was found that fumes from either the C-3 explosive or the cement used to adhere the sight to the top of the mine corroded the plastic mirrors, rendering them unusable.

In the end simple peep sights were adopted, which was later replaced by a knife blade sight.