I have no experience with these style of machines, so offer the following just as food for thought or avenues for investigation.

It's not unusual to have the earth of a system commoned to the 0V.  With two independent power supplies as you have, notwithstanding it looks like one has been added later or repaired, it would not surprise me to see the 0V of both supplies and the earth tied together.

You could most likely confirm this just by visual examination of the wiring.

If the second supply has been added as a bodge to beef up the supply they would almost certainly want to have a common 0V.  I can't read the labelling on the silver psu, but I'd be verifying how it is wired in relation to the black one. Maybe as the +12V on the black psu is not wired it has failed and this is the function of the silver one, to provide the failed 12V?  The black "looks" more original to me.  What do the terminal labels of the silver psu read?

With the power off and unplugged I'd remove the yellow / green wire to the PG (protective ground) terminal on the black psu and measure the resistance between this PG and the terminal labelled GND.  Try this with the silver one too if you can find 0V and ground (earth) terminals. I'd expect close to zero ohms.  (The two ohms mentioned elsewhere is the wiring standards maximum resistance between the earth stake and any other point on your house wiring, so not that directly relevant here) Maybe then report back and the results of your visual wire tracing.

In any case cleaning up the rusty earth cannot do you any harm - but WORK WITH THE POWER OFF! And ensure the outlet you are using it on is rcd protected. This is good advise for any of us amateurs playing on this forum.