How improve temperature sensitivity of HP 721A DC Power Supply voltage reference?
Maybe replace Q4 and the reference diode with a super-cheap (pennies in quantity) TO-92 TL431 and two 1% resistors set up to maintain the -6.9V voltage where the emitter of Q4 is connected. The 560R resistor can stay if it is 1/4 W or better, just remove Q4

Resistors would be something like 4.99K and 2.80K.

In other words, connect a shunt regulator between 0V and -6.9V nodes, and use the existing 560R resistor to pass 16mA through the shunt regulator. It will typically drift about 50ppm/°C. (the 560R resistor will be on the other side compared to the below schematic)



Of course there are much better shunt regulators available but I'm not sure the difference would be worth it.