Lensatic compass (Andover, MA)

Lensatic compass, brand name Engineer.
Round black plastic case is hinged but I can't open it.
Found in a box with unrelated stuff; no instructions with it, not sure how it works.
Google says that lensatic compasses are more accurate than the standard ones.
Free to whoever wants to figure it all out.
Maybe a Scout troop, compare with regular compasses? Reply by Craigslist email or call; landline does not take texts.