Ofna front lower amr holder is an optional part and provides the holes in the same positions as the 'B' lower block. You must use this part along with the upper 'B' block, either the plastic or aluminum version!
The 'B' blocks make the rear of the front arms sit higher than the 'A' blocks, which descreased the front kickup and also decreases the amount of front caster at the steering hubs. This will provide more corner entry steering but less corner exit or on power steering. If you need more steering entering corners on power, or your buggy wants to break loose on long sweepers, try the using 'B' blocks