Hello,
The 1841 has two WIC slots as well as two built-in Fe ports. Looking at the "Show inventory" out of the show tech you provided, it shows that there is a serial card in both of the available expansion slots. It looks like the router is filled-up.
------------------ show inventory ------------------
NAME: "1841 chassis", DESCR: "1841 chassis, Hw Serial#: 336740761, Hw Revision: 7.0"
PID: CISCO1841 , VID: V05 , SN: FHK112516M7
NAME: "C1841 Motherboard with 2 Fast Ethernet", DESCR: "C1841 Motherboard with 2 Fast Ethernet"
PID: CISCO1841 , VID: 7.0, SN: FHK112516M7
NAME: "WIC/HWIC 0", DESCR: "WAN Interface Card - Serial 2T"
PID: WIC-2T= , VID: 1.0, SN: 24046985
NAME: "WIC/HWIC 1", DESCR: "WAN Interface Card - Serial 2T"
PID: WIC-2T= , VID: 1.0, SN: 32986640