As much as I love ar's I still don't know some basic stuff.

So that being said, my 20" Colt A2 has a "cal .223" marked lower and a 5.56 marked barrel.

What should I take from this? Why the discrepancy between the two?

Barrel markings will tell you what can be shot through it.. 5.56 is safe to use.

As for the markings on the lower… my guess is that .223 marking made the gun more politically correct.. easier to sell & export

When Colt introduced the AR to the civilian market years ago, they marked the receivers .223. From early on, they tried to differentiate between military and civilian models. I can only guess stamping them .223 was one way to reinforce it being a civilian model.

The stampings are just stampings and they mean very little. There are manufacturers that will stamp their guns 223 or 5.56 for a variety of reasons. Some manufacturers will stamp 5.56mm as opposed to 223 to eliminate confusion as the consumer has wrongly been told that if they see a "223" stamping anywhere it would be dangerous and irresponsible to fire ANY 5.56mm through the gun. Some manufacturers stamp their 5.56mm (or Wylde's) as "223 Remington" to make it less militaristic and more consumer-minded, this is usually political as there are many countries that strongly regulate ownership, import, and/or export of "military style calibers".

Typically barrel stampings are important (though I'd argue if the rifle is 223/Wylde/5.56 marked it is less so), but receiver markings don't matter at all as various other calibers can typically be used (are often are) with one particular frame or receiver.

Makes sense. I was hoping I didn't have a random lower on a random upper.

Thanks

Good to go. I have a few older Colts marked that way. One is a pencil barrel Colt 6520 A2 carbine that has the “restricted law enforcement” rollmark on the right-side mag well. Made during the ban for law enforcement but still marked .223 on the receiver and 5.56 on the barrel.  The barrel is the important mark, that tells you what the chamber is set for.