Inspired by a folding ruler.

I treated myself to a mooch around the Antiques Emporium today. I enjoy the trip down Memory Lane - 1950’s furniture and kitchen equipment we discarded decades ago.
I’m fascinated by small wooden tools. This folding ruler caught my eye. It looks as if it is made to fit in a breast pocket, or neatly in the hand. But what is the groove for? Is there a part missing? Perhaps that’s why it only cost me £2.