"masking effect"

Greetings,

I am not a printing professional, but a graphic designer and an amateur photographer, so excuse me if I come across as ignorant to some extend of the basics. I do know the basic workflow of offset printing though.

My question has to do with what I perceive in old (pre 90's) color printed materials, like books and magazines. It looks pretty much the same as what you'd expect from traditional darkroom masking (whether for contrast reduction or unsharp masking), which produces a certain negative halo around objects. For example a bright halo around a dark area and vice versa.

Here is an example of this:

http://s.petrolicious.com/vintage/2013/4-5/porsche-brochure-7.jpg

sorry, couldn't find more examples for now, but I've seen this in a lot of old printed materials.

There are two ways in which this can happen: intentionally on the part of the printer (like in the case of masking), or accidentally due to the technical limitations of some part of the process. The second is true in motion picture optical printing effects, especially in 50's and 60's, in those shots featuring an optical transition like fade. In photography this can be the caused by the "creep" of development inhibitor chemicals around the developed zone in the image.

But I have no idea why is it seen in old offset printing.

So my question is, is this a result of intentional masking used by the person exposing the negative (to be used for making plates), in order to, do what? Sharpen the image, reduce excessive contrast?

Or is it a byproduct of some part of the process, like plate etching, or something?

Thank you

P.S. by the way, what was the method for increasing saturation like this, for example on old postcards full of that unnatural cyan skies etc.?