There is bittersweet irony in the K-231-A. From the Muirhead journal (1966) posted above by lowimpedance:
![](https://www.eevblog.com/forum/metrology/muirhead-std-and-ref-cells-all-catalogue-info-uploaded-!-(ayce)/?action=dlattach;attach=284874;image)
These were the last 3-4 years of a dying technology - trying to beat the Zener diodes coming from the semiconductor fabs.
To complete the Irony - the K-231-A (1967) has two Zeners in it to set the control threshold for the temperature regulation.