We must remember that the pictures are an illustration to get across a sighting concept. The illustrator himself may not have been a hunter.

I would assume that the sighting is to go through the centre of the bead, this is how nearly any reference in the old literature seems to have the V and bead used. The idea of the point of impact being at the top of the bead, or a "six oclock" hold is an Americanism from military style competitive target shooting in the 1920's using a peep sight and blade front sight; I think Townsend Whelen most responsible for it.

For hunting with express sights, shooting through the centre of the bead is much more natural when under stress and I sight all my rifles that way, with point of impact through the centre of the bead. For people who think that the bead will blot out the target at a distance, these sights should be used with both eyes open.

The sighting method above using different positions within the V for the bead is a 19th century convention leftover from black powder rifles (like the buckhorn sight) order to handle the high parabola of the trajectory for those old cartridges.

I remember WDM Bell counselling not to "monkey around with the from bead" by taking a fine sight etc, because in lower light the shooter will show more of the bead in the V simply because he can see it better...

I have done a lot of experimentation with open sights. (It was illuminating when I discovered the flat top rear sight was just as accurate as a shallow V or notch. You simply balance the bead over the top of the flat rear sight, so that it is in the middle. There is no notch or v at all, just a straight bar of the same width. You wouldn't think it would work at all, but I recall my first three shots at 100 metres made a one inch triangle group. Which was what that rifle would do with a four power scope.)