I noticed something the other day about Finnish M91 sights. I've been trying to get into them, seeing as how these old, clunky beasts were in fact the ones used to rack up over 500 kills in the Winter War by history's greatest sniper. I haven't had a chance to test it yet, but it seems to me there is a second set of sights hidden on top of the first set.
The first set is, as you know, an old-fashioned tangent sight system with an arched rear tangent and a very simple blade front sight. Higher on the Finns than the Russians.
To aim you try to fit the very thin front blade in the equally thin rear notch, a feat that works fine at the range but not so great in a hurry or in bad lighting.
BUT if you will notice the two sides of the riser bracket the sight picture thusly:
]___[
If you ignore the notch and lift your head up slightly, the front blade fits between the RISERS, with its top equal to the top of the risers:
]_I_[
Now my bet is if someone were to aim at the target like this the bullet would have the same zero as the tangent's setting, because the barrel isn't moving just the height of your eyes. It is certainly much easier to get a clear sight picture with the blade fully exposed and no fine notch to fit it in. You just center the post between the risers and bang away, using your eye's natural desire to find the center to aim instead of deliberately trying to "thread the needle" with the fine sights.
If I'm correct, this is similar to the secondary sighting devices utilized in the 28/30 and M39 sight to center the post. And it should work particularly well with the Finns since they have a nice high front sight. Clever boys they were.