This is a voltage reference based on a well-known Chinese "LM399" module which actually appears to be staying pretty stable over time. How precise the numbers that are hand-written on it are is quiestionable, but they appear good enough and they can always be verified when there's a chance to use a precision voltmeter. Most importantly, it can definitely be used to check if the DMMs are still all right, which is its primary job.
Inside is a linear power supply and mode selection circuitry that uses relay contacts where a jumper was used on the module's pin header to select one of the six output voltages.
Usage:
And this one's used literally every time I need to try something out on a breadboard or quickly power something up otherwise.
Nothing fancy, just an isolated transformer -> bridge -> caps -> one of those well-known Chinese DC/DC modules.