Great to hear you received the pistol!
I am sure RIA will take care of you just fine to fix the mag issue.
I have 7 mags all together for this, the original one and 6 Kimber PN#1100018 mags. I was told by the guy at RIA that these were the exact same mags they issue with the gun (just that they do not say Kimber on the bottom). I got a good price on buying a handful of those on FeeBay so I jumped on it.
As mentioned previously I removed the spacer from the spring and made them all a 15 round mag like they are supposed to be.
They all lock in good and drop free except for one of the Kimber ones, it is tighter and drags a little more. Don't know if it is thinker or warped a bit.