This is a fine example of a cabinet maker's/ carpenter's hand made scribe or gauge.
It's a quality hand tool simply designed, made from mahogany.
I would think it's Victorian, 19th century.
It has the names of past owners carved into it, showing it's history.
There's a sliding handle which is beautifully shaped for comfort; the other side has a stepped guide, that's 6inches in length.
The whole length is 14.5inches.
This is a precision marking tool used by skilled craftsmen and can still be used today as it's an essential hand tool when working with wood.
It's in wonderful original condition!