This is an interesting naval slide rule made for the NATO. It was produced by Blundell from England and is called the Bearing Rate Slide Rule, mark 1 – model o. It has a patent number and a NATO number on the rule itself. There are no instructions but it comes in the original wooden box (1 lock part missing). It is an unusual heavy all plastic construction with large metal and plastic cursor. Everything is in great shape with some minimal signs of age and use. The slide rule measures 40 cm or 16 inches wide. Check the attached video on how to work your vintage man tool : slide rule operation instructions!