I would guess that the little lens in front might be part of a built in optical slave trigger.

And as for daytime use, you determine the aperture to use with the flash from the table at the back and then adjust the shutter speed to give appropriate exposure with the ambient light - remembering that with most focal plane shutter cameras you are severely restricted in which shutter speeds may be used with flash.

Your GF670 will most likely synch at all shutter speeds, but the FM2n won't.

You can adjust to taste the ratio of ambient light exposure to flash exposure. I tend to prefer normal exposure from the ambient light, supplemented by flash that is one stop under-exposed.