I recently bought the Arduino Starter kit that comes with the Uno Rev3 I thought. In reviewing how to use the Yún Shield it looks like I don't have the ICSP headers for the ATmega16U2 so now I am wondering if the board sold with the kit is a different board.
Actually it looks like the ICSP headers are missing on the Rev3 board that is sold standalone.
I guess the question then is how would I perform this step
as described here
Yeah, for some reason they didn't populate those headers on the ones manufactured by Adafruit, who makes the arduino.cc's Uno since the .org/.cc split. That's unrelated to the Rev3 thing, which refers to different hardware revisions.
You can easily solder a header on there. I've used a header with the pins bent in a little to get a good contact inserted in the holes without soldering for a temporary jumper in a different application but I wouldn't count on it for long term reliability, much better to solder for this application where you need the jumper left in place and getting good contact.