Riku's naming convention (KRB-#X) is simpler than it looks.

1. KRB = Kusaka Riku Build
2. # = Total number of Gunplas
3. X = Variation of a specific #.
4. The letters correlate to number sequence. A=1st, B=2nd, etc

So in the case of Metal Angel (KRB-7N). The 7 means it's the 7th Gunpla Model he's purchased/built. N means it's the 14th variation/redesign of said model.