The Weaver system requires the base to be installed and after that the rings and scope can be removed leaving the rifle usable with iron sights. The PE and PU do the same thing, but the Weaver also has the advantage of being usable with any 1" diameter scope.
It is a very clean and low profile installation that leaves the rifle "looking right" with either the scope or the irons, and sets the scope in a good position to use without needing a neck like a giraffe. I have modified a few in the last 3 years to work, but it's a bit of a pain, and it sure would be nice if Weaver would just make the base for the Mosin Nagants and save us all the headaches.
The system is very good.