Tony,
I'm a bit confused. A 9.5" wide rim with centre (zero) offset could not have 4.75" backspacing. A 9.5" rim with zero offset would have 5.25" backspacing. Conversely, a 9.5" wheel with 4.75" backspacing would have .50" negative offset.
As far as I know, all wheel manufacturers calculate offset and backspacing the same way as pictured in the diagram below.
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