R1A1 was Century playing on the South African R1 designation, the kit your rifle started as was an L1A1 (Brit or Aussie depending on the markings). The metric receiver is just what it was built on of course. Birdman is the Gunsmith .... The F1 flash supressor is a limited production variation to the standard L1A1 flash supressor, it also acts as a MB to a degree, much shorter than the origial issue version while retaining the working properties, it also allows the use of the standard Commonwealth pattern bayonets (sould you need to put "cold steel up 'em") ....