A strap or chain used to attach a watch to a pocket.
Formal fobs utilized a chain. Equipment manufactures used strap type fobs to promote their products by attaching charm like symbols and logos to the strap.
They were handy to extract the watch from the 5th pocket of a pair of jeans.
When wrist watches came into vogue, the fob gave way to T-shirts and hats as the promo items of choice.
My collection includes about five hundred fobs. The first two sections of the case below contain concrete company fobs.
On top the case is a collection of concrete company lighters.
Here's a fob from a Massachusetts company,
Rowley Ready Mix, with a watch attached.
and here is how it's intended to be worn.