distance /measuring /map template use

[FOBC]

Ive had this piece for years just wondering what it was used for? possibly artillery or just map scaling never seen another one of theses its marked bvu W&A 278 has an eagle symbol above tried my best to try taking a pic has a stud on one side purpose of it?? . Was told it was from the Stalingrad area any info would be appreciated as i try to document the item