M3 (3 mm) Carbon Steel LN Key Bolt

An "LN key bolt" is most likely a socket head cap screw (or Allen bolt) that requires an Allen key (hex key) to be tightened. The term "LN" doesn't refer to a specific feature of the bolt itself but could be a typo or shorthand for a bolt with an internal hex (Allen) socket, as seen in the context of Allen keys and bolts.