The fuse box is probably embossed with info, usually >> MAIN, HEAD, TAIL plus the amps & wire colors.
For stuff like this you can find the part number at Partzilla, then search EB. EB pics are handy, especially NOS.
As mentioned before, the KZ400.com site has a wiring diagram for your motorcycle.
If you need fuses, verify & measure yours before ordering. On my 81 KZ650-CSR the main fuse box fuses are 25mm in length, while the accessory fuse box fuses are 30mm. 25mm = 1" & 30mm = 1-1/4". AGC & AGX. I forget which is which.
1981 KZ650-CSR. Competition, Sports & Road.