Does anyone know what this is?

[The Dad Fisherman]

A key for a Security guards's clock. as they make their rounds they are at different locations along the route. they insert and turn, it imprints a letter that coresponds to that location on a Time wheel. that way the company knows you did your route and at what time you were at each location.

[FishermanTim]

That's the best answer by far!

[nightfighter]

That was my initial thought, but the key shown is too simple to differentiate locations in watchman's rounds. That looks like the pin that holds up the tailgate to my 4x8 trailer.