Check which cards are currently fitted inside. See the FAQ for card info. Cards are interchangeable. Low output MC carts need NA323 cards, either S or K.
Earlier Naim pre-amplifiers used interchangeable daughter boards. While the very earliest NAC12s did not use daughter boards, most were updated, and the early versions of the NAC12 and NAC22 used NA1xx and NA2xx boards that used knife edge connectors but were superseded by pin mounted NA3xx boards.
The NA3xx boards that can be found and used in later NAC12s are;
NA325 - Switching Relay, switch-on output delay x 1
NA321 - Main gain, output drive x 2
NA324 - Input Buffers x 2
NA322 - MM …