Every once in a while you have to build a rig that has a strong side or rotational load on the base plate block and this can occasionally cause the base plate block to loosen itself up - not good. This base plate solves that issue. Both sides are grooved with a channel that is .040 deep to prevent base plate blocks from unscrewing themselves when under a rotational or side load. One side is groove to accept the 750 series blocks and the other is grooved to accept 500 series blocks.