Actually the one I ordered through my shop just pitched up today... I wanted it because, well, I wanted a Gov't Carbine.
I don't think there is much rare about them, most people want a flattop to mount optics on. In fact, I had originally bought this to sell the upper and use the lower to register for an SBR. I like it and am going to keep it for my collection and just SBR something else.
Mine has the LE/MIL warning, which I am told Colt quit doing. All the distributors are freakin' backordered, luckily another shop cut us a deal.
I'm actually suprised Colt is still turning them out considering a flattop is a better platform.
To answer your question... I don't think Colt makes as many as they do the 6920. I think the people want a 6520, well, they want one. I think a majority of the AR buyers prefer the 6920.