I bought the Armalite 14.5" upper a while a go, and I had a spare Colt Canada lower for it at the time, but eventually, that lower got appropriated for another build and the 14.5" upper ended up in my spare parts box.
I've been keeping my eye out for a used Colt USA LE6920 lower, but haven't had much luck. There was a time when there were plenty of LE6920 lowers in EE and everyone wanted to trade them for a Colt Canada lower, but now it seems like it's the other way around. The market seems to be flooded with Colt Canada lowers, because now you can get them stripped for about $200 fairly easily. I ended up getting a Colt Canada lower off a buddy, along with a Colt Canada lower parts kit and ambi selector switch.
A comparison between the Midway USA ambi selector (top) and a Colt Canada ambi selector (bottom). The Midway USA one isn't bad if you're looking for something that sort of looks issued. The main difference that really stands out is the full length lever on both sides, where the Colt Canada (and Colt USA as well) have a shorter lever for the right side. The Colt Canada one also comes with a washer/spacer to fill in the gap, where the Midway USA unit does not. The price difference is the Midway unit is about $20 USD and the Colt Canada unit will set you back $135 CAD
The lower came with a Magpul trigger guard, and though I really like this trigger guard, standard M4s don't come with them. I knocked it out and threw a standard miil-spec one in.
Again, these roll pin starters are awesome (see my previous lower builds).
I actually yanked the Colt Canada ambi and threw it onto my CANSOFCOM clone build, but I doubt anyone would notice such a small detail. This lower ended up with one of my Midway USA AR Stoner units.
The lower didn't come with the receiver extension, but I happen to have one in my spare parts bin, so on it went.
I also had a spare Colt Canada butt stock in black. I really wanted another Colt USA N1 stock (see my Blackhawk down build), but they go for about $79 CAD in beat up condition, if you can even find one. I've also got a spare Colt USA waffle stock I can swap onto it.
Anyway, this new lower will do until I find another LE6920 lower.