That last one is really interesting. It looks like it was made right during the transition to the M16A4. The lower was probably already marked as an A2, but they shifted over to A4 production and applied an A4 serial number to it (much higher number series). Thanks for posting that one too.
I found the original image through your link - thanks. I'm reluctant to post the linked image in the pinned A2 lower thread in case it gets taken down eventually, so I'll probably upload a copy directly to the lower thread. I'm not sure how much this site compresses images when you upload them, but I'll try for as high resolution as possible.
By the way, it's really interesting to see the FN CAGE code on the hammer in the first photo you posted. I don't think I've ever seen that before, but of course, the hammer's usually not visible in photos.