1920-30's waistcoat with a "loop" between buttons

[Retrospective]

Hello!

I recently bought a 1920-30's frock coat with a waistcoat (both are probably tailored in Japan).
On the waistcoat, there is a mysterious small "loop" attached between the buttons (please see the attached files for details).
Does anyone know what this is for?
I did some research but I could not find any information about this "loop".

Thank you.

 

[RossRYoung]

Most likely a pocket watch chain