Hello,
this additional PCB on the bottom side contains a protection structure, I think for the DCV path, which was before assembled on the top side of the 34465A main board.
The previous 34460 main board seemed to be in most parts identical to the latter one, only that some components were not assembled (see review activities when Dave initially made a teardown of the 34470A in 2015).
In this security bulletin:
https://www.keysight.com/upload/cmc_upload/All/Truevolt_DMM_Product_Declassification_Security.pdfit's mentioned on the last page, that Microsoft Windows® CE 6.0 R3 is used inside these instruments.
Support by MS was already finished in about 2018, and EOL is announced probably for next year, like CE 5.0, but I found no official statement yet.
That means, that any time soon, KS will not be allowed to sell new devices with these old O.S.
I doubt, that KS in between was able to move to direct successor O.S. like Windows Embedded 8 or 8.1, if that would physically possible, actually.
Rumors that KS is already in preparation for a new O.S. were already expressed, I think on our MM 2019.
Hopefully they get this done in time, as I assume that the whole line of TrueVolt instruments, like also Generators, sources and PSUs may be based on that SW platform.
Frank