70% brass with 30% zinc mixed in.  Which companies use / don't use this mixture?  Any advantages / disadvantages?  What does Lake City use?

Thanks.

Here's some info on brass composition:
http://www.accurateshooter.com/technical-articles/x-ray-spectrometry-of-cartridge-brass/

“Cartridge brass” is a (generally) standard alloy of 70% copper and 30% zinc.  That doesn’t mean all cartridge makers use exactly that alloy.  Varying the proportions gives the alloy different characteristics - mainly on a spectrum from soft and springy (more copper) to hard and tough (more zinc).  Other components like tin tweak the metal further for specific (and probably very arcane) reasons.

The Accurate Shooter article gives a couple of excellent examples of how different the alloys can be.  With that said, both Winchester and S&B use an alloy that’s pretty close to the classic cartridge brass.  This alloy was developed (empirically and long ago) to provide a very good compromise between springiness and the ability to harden parts of the case sufficiently to get the case to do what it needs to do.

I think that the variations between different alloys may be why annealing some cases leaves more or less of a discoloration - according to the AS article, some alloy ingredients change the visual characteristics of the brass, and might affect the degree of color change seen with even very precise annealing processes.

Essentially nobody.  A pure 70/30 copper to zinc mixture, with no other alloying elements, is today pretty much 1) a notional concept, and 2) an historic artifact.  Cartridge makers order alloys from the mill that have certain mechanical properties to fit the cartridge cases they are making with the tooling and processes they will be applying.  Unless they have records from the manufacturer, nobody is going to able to tell you for sure that Federal bass is always X/Y while Remington is always A/B.

As to advantages and disadvantages, it would depend entirely upon what you considered to be advantageous and what you considered to be disadvantageous.  More copper makes the case softer.  More zinc makes it harder.  Is hardening due to cold work a concern?  What about how readily the case seals the chamber early in the ignition process?  Do you reload; so that case life is a concern?  What about corrosion resistance during storage?  The particular alloy determines these characteristics.