Yep, that is a 63 wheel cover as the 64 had an "R" instead of the Buick shield. The cover is aluminum and bolts to the wheel with studs and nuts on the back side. The center section comes off like a Chevy Rally wheel.
You mentioned the name Bill Williams, that was my tie in he oversaw the design of both. Do you know another thing about them, in their body construction that was the introduction of this process? (at GM at least but I think they were the first)
It's your floor BB