The 'international' FM band is 88 to 108 MHz. So that no radiation goes outside this band, the lowest and Highest frequency is 0.1 MHz inside the band limits. This means that the lowest station frequency is 88.1 MHz, and the highest is 107.9 MHz. The spacing of station carrier frequencies is at 0.2 MHz intervals so each station has a carrier frequency which is at an ODD frequency.
On the AM side, the usual spacing of station carrier frequency was 10 KHz; However, in Europe, the spacing was reduced to 9 kHz. So there is an arrangement that for the US, the stations are selected on a 10KHz spacing for AM and 0.1 MHz for FM. With Europe, the station spacings are 9 KHz for AM and 0.05 MHz for FM. This is a function of the frequency selection process in the receiver. My guess is that the Optimus, being for the US market, is NOT SELECTABLE, but I may be wrong.
You ask about the wires from the primary side going to the electronic board and I dont know. Is it possible there is a panel lamp (on/off indicator lamp)? It is also possible that the mains lead goes to the radio receiver and is an 'internal aerial' for AM and FM.
It may also be possible that the leads go to another power outlet to be used for an attached turntable or tape deck perhaps. When the mains switch on the OPtimus is OFF, then there is no power to the turntable.
Thats all I can suggest.