Connections for the NAC12S are marked on the case, altough I can't make them out in your photos.
Other, clearer pictures of the NAC12S on the internet show only three DIN sockets, and what looks like a combined power in/pre-out socket (with power amp providing supply). Yours appears to have a fourth socket, which may be a dedicated power in. But I can't see from your photos. If so, your third box may be a PSU for the pre-amp.
http://www.canuckaudiomart.com/details/649410738-naim-nac-12s-ser-1396/images/1773156/
I can't see any phono inputs on yours, and the connection ident label is in a different position, so maybe yours has a DIN inpit for the turntable. Yours looks more like this:
https://forums.naimaudio.com/topic/bolt-down-nac-12-nap-160-coming-my-way
which looks to have been modded to split the pre-amp out to phonos, and the original pre-out/power in socket replaced.
Loving the old skool 'gram' input...
Other internet pictures of the NAP120 also show a different rear panel, with speaker sockets on the RHS, rather than centrally like yours.
I note a twin signal cable on the DIN socket of the power amp; this might suggest it is signal in only.
Some judicious use of a multimeter to check the connectivity and voltages on various pins may help. Or a post on the Naim forum, which seems fairly active.
picked these items up a few weeks ago but have no idea what connects to what.
