I think usually you’d have a separate grenade sight, which can be folded down when not in use

(although on some older rifles you’d actually just have to read a chart and calculate angles and ranges yourself)

I assume here the grenade sight just happens to be right next to the regular front sight, and has been folded up in the picture so it obscures the normal sight. I’d guess this bit right here must be the folding mechanism?

So there’s probably a normal front sight right behind it, at the height it should be, it’s just not visible at this angle