I had the rattling power button issue on my 311K version and requested an RMA on Dec 17. The 311K version replaced my first 310K version as that phone had the so called dim yellow tinted display that looked awful compared to my N4 in terms of whites. Received replacement phone yesterday and to my surprise it was a 310K version. Comparing them side by side, it is obvious that the 311K version has the slightly larger speaker holes and buttons. Funny enough, even with the smaller speaker holes and thinner buttons, the 310K version has slightly louder volume output (trivial) and the power button feels better attached to phone and has a more solid click to it when pressed. Also, I find that the 310K version has a less sharp bottom edge where the glass meets the bottom bezel when you slide your finger across it. That being said, I do not think there is any difference in performance attained from the button sizes and speaker holes. The rattling button issue is probably related to how meticulous the person putting them on the phone was. The speaker hole size really doesn't make any difference for volume loudness especially since the 4.4.2 update.