PPM LTD Model 410/411 Voltage Reference

[TonyGreene]

I am looking for any information on a PPM LTD (England) Model 410/411 Voltage reference. 

The box is one rack unit high for a 19 inch rack.  On the rear it has female banana jack showing a zener at 6.25498 volts.  It has 1000 and 100 volt inputs to a divider that outputs 1 volt.  It has 10V, 1V, 100mV, 10mV, 8mV, 6mV, 4mV, 2mV, 1mV and 0 volt outputs, along with a guard and sense banana jacks.

This unit shows it was calibrated last September 1987.  A transformer inside shows a built date of 1979.  The unit uses 0.005 percent green wirewound resistors and the older "TO" can style op amps that are OP07.

The unit used 14 "C" cell nicads, which 2 cells supplied a negative voltage, and the remaining 12 cells supplied a positive power supply voltage.

Thanks