ACC(essory) ? not acronym ?It is for the NAGRA tape recorder (6-pin)
"39. ACC (accessories)
CCIR NAB
1 = Input signal Ground
2 + 3 = Balanced signal Input
1. No connection.
2. Ground
3. Line input, current drive. Minimum source impedance 47K½.
Current for obtaining 0 dB at maximum sensitivity = 3.73 ìA.
4. Tape speed correction signal input.
5. No connection.
6. –10 V stabilized voltage"