The steel pad has its purpose to be sure, it sits in between the throw lever actuated "wheel" (for lack of a better term) and the under rail surface. The pad sits loose in that it's not under spring tension, so when you put the mount on, you have to make sure the pad is pushed towards the outer rail or the mount will not properly seat on the rails because it's in the way.
LaRue's mount does not have the pad so there's nothing to push aside first. On the other hand, LaRue's "wheel" comes directly in contact with the rail's under side, sometimes the friction can cause wear on the finish as it did mine. But in my case, I'd be the first to say the B&T mount is kind of crappy
LaRue on left, ARMS 19S on right. Note the steel pad is on the "inside" position and will need to be pushed towards the right before being put on a weapon again.