Ultra Precision Reference LTZ1000
This is released as Open Source / Public Domain. There is no licence.
https://keen101.wordpress.com/2013/02/12/hpagilent-03458-66509-7v-dc-reference-clone/
HP / Agilent / Keysight 03458-66509 7V DC Reference Clone
replacement boards for the HP/Agilent 3458A multimeter. Ultra Precision DC Reference board which uses the LTZ1000.
It looks like someone else beat me to it: https://github.com/pepaslabs/hp-03458-66509-clone
I previously released a design for the cap that goes on top: