Based on my experiences with civil war era items... the "US" marking was stamped onto equipment which had been purchased by the Army AND had passed an Army Inspector's quality-testing, certifying it had been "accepted" for service. The US Navy put a similar marking on equipment its Inspectors had quality-tested and accepted, except that the Navy's mark included an anchor to distinguish its marking from the Army's mark.
What that means, in summary:
Your Stanley pocket-level is different from its many identical brothers which were sold to the public in Hardware stores only in that yours was purchased from its manufacturer by the US Navy, inspected for quality by an official US Navy Inspector, and accepted by him for issue to Navy personnel.
I've seen many civil war era (and later) items of US Navy equipment which bear essentially the same marking seen on your pocket-level. Unfortunately, I don't have a photo of an example handy. Perhaps somebody here can find such a photo faster than I can.
Edit: I got a notification that NOLA_Ken's reply came in while I was typing mine. Consider my post as confirmation of what he says ...it is a US Navy Inspector's acceptance stamp. Let me add, for other readers, that although this Stanley pocket-level is 20th-Century era, the US-with-Anchor acceptance mark dates from around the civil war era to the present day.
