Dave,
thank you very much for that great video!!
You can literally feel and see, how much of analogue magic was designed into this instrument..
All these guard rings, isolation slots, and shielding methods, including that really ingenious low noise DC/DC converter..
They put in all of their know how about Keithley electrometers and Fluke calibrators, obviously..
And one of the former HP3458A engineers works for Keithley since a few years..
So there's really where the brain is inside, not in the Keysight counterpart.. that's very obvious, if you simply watch both videos.
OK, some remarks about the voltage reference and the thermal shielding..
This LTFLU package is supposed to be the regular one with 4 pins only, (judging from the type designator printing and photos from this device), so there is no heater inside the TO99 package, or not used, if there is any heater implemented on the reference chip. (as DiligentMinds always speculates)..
The white (ceramic ?) collar you see around the LTFLU, seems to be the exterior heater, it's named 'H2', obviously.
Its temperature might be controlled by the 2nd OpAmp , maybe.
The plastic or metallic shield around the whole assembly, also the one on the bottom side, also serve as a thermal isolation, so that the whole interior may heat up .. there's a LM35D temperature sensor also, maybe to measure if the whole assembly has been stabilized.
On the bottom side, I assume that there are 4 stable, (leaded?) resistors, which are always needed for the topology of this Reference Amplifier, aka LTFLU / SZA263 / T.I. device from all Fluke calibrators and voltage standards.
Anyhow, very impressive, and maybe, some day, Keithley will replace Keysight as the top notch DMM manufacturer..
Frank