Maxwell,
ALL stock W123 5.75 FOG lamps came with only one filament. therefore you can only have low beam on the fog lights (stock arrangement).
If however it was modified by someone, it would possibly have hi/low beams......
Take a look at the bulb in a fog light. If its a three pin bulb its been modified. Normal fog lights(W123) have a H3 bulb with only one filament, it should have a flex lead that is one terminal the other is the ground. Let us know then we can advise you better Thanks
Dieseldiehard