I finally found the dilemma. It is like an 1N5388 200 volts zener. It is on the circuit to keep the voltage down to 200 volts at the pass transistor at turn on. My friend has an amp with the same reg. There is 3 of these mystery diodes in the reg. When turning on the amp, at the 3 diodes, the voltage reach 200 volts and lower to near 50 volts when the reg is hot.