Colt half moon BCG

[Deserthunter]

I'm confused regarding the old Colt Sporter AR-15 half moon BCG, which were semiautomatic.
Just where is the half moon or C cut? Is it on the bolt or the bolt carrier? Usually these Sporters had a sear block in the lower receiver, which prevented the installation of a full automatic BCG.

To replace the half moon so the Sporter is not in the semiautomatic mode following removal of the sear block, does one replace the bolt or the entire bolt carrier group?

 

[MistWolf]

To convert an AR to full automatic, a full auto carrier, full auto trigger and full auto sear must be installed. To install the full auto sear, the lower receiver must be drilled for a third pin. I believe the lower also needs machining done to the shelf area.

To install a full auto carrier in an early Colt with the moon cut, the sear block needs to be machined for clearance.

 

[Project355]

Early Colts didn't have the block. That didn't start until the late 80s or early 90s