A-V-8R - that's a new one on me! Do you mean body-gear when you say centre-gear? The only positions for the gear lever that I know are "up-off-down". The earliest 747 I flew had it's first flight in May 1970 and I have not heard of a section of landing gear usable as Speedbrake. Are you sure that you are referring to the B747? Of course it may have been customer-specific and perhaps "modded-out" at a later date.
To answer another query, the body-gear has steering capability - it turns the wheels in the opposite direction to the nose-wheel, thereby reducing the turn radius. It is selected by the crew on the -100/200/300 and automatically on the -400.