Symbol 5070 from IEC 60417 - Title/Meaning: Monophonic Function/description: To identify the ""mono"" position of a ""stereo/mono"" switch and on equipment to indicate that it is suitable for monophonic sound only.
Additional information: 
Published in: First edition, 1973
Own work using: iso.org/obp/graphics/grs/b934f873-fc0b-4df3-b89d-4363aa798e0f 200.png
Vectorization:  Mrmw