Hosa Stereo 3.5MM TRS (F) - 1/4" TS (M) Adaptor
Jack's Guitarcheology is a distributor for LPD Music, which distributes Hosa.
From Hosa:
This adapter is designed to adapt a mini stereo phone plug to a mono phone output. It applies mono equally to stereo left and right.
Connectors: 3.5 mm TRS to 1/4 in TS