Hosa GPM-179 Adaptor 3.5 mm TRS to 1/4 in TS. This adaptor is designed to adapt a mini stereo phone plug to a mono phone output. It applies mono equally to stereo left and right.
This item will come out of box from large bundle... variety of makes and models. Mostly genuine Yamaha. First pic is for example
The Hosa GPM-179 is a 3.5 mm TRS to 1/4" TS Adaptor used for audio connections. It is a product by Hosa in the category of Cables, Snakes & Interconnects, commonly used in musical instruments and pro audio equipment.