An L + R signal formed in a mixer, used to drive a “hole-in-the-middle” speaker.
"I sent in a priceless recording of my deceased Father-in-law recorded on his ship in WWII. He was receiving a Bronze Star Medal for his heroic efforts on board. The old 78 recording had so much static you could not make out much of what was said. My husband will receive this BEAUTIFULLY restored recording as a surprise for his 1st Father's ..."