There's already been the 2018 AN: 'Calculating Uncertainty using Digital
Multimeter Ratio Measurement Techniques', containing pictures of this black, overhauled 3458A.
KS need RoHS compliant electronics, as the previously stocked EU market EU3458A may be depleted.. and also this >30 years old design now has probably many terminated components inside, so that it is becoming more and more difficult to make bodges, or the longterm stock for old components is also depleted, meanwhile.
So what we have heard end of May, that a transfer design was done on all PCBs, from through-hole to SMD technology, updating also many component types.
KS has no real development team anymore, so the schematic, software and so on is untouched.
The rerouted, smaller PCBs should be backward compatible, so that new spare PCBs might also go into old instrument.
Hopefully, the new, tighter packed layout did not let the magic escape, that may be under investigation, being the reason why it's not been released, yet.
Frank