You are looking at this yellow gold Ref. 6594 from Antiquorum. You say the case is not original (or over polished?), and that the crown and buckle are not original.
When comparing the casebacks between this watch and your own white gold Ref. 6394, you noticed the brushing on the case backs are different.
- The YG Ref. 6594 case back is circular brushed
- The WG Ref. 6394 case back is linear brushed
So your question is "Is the case back of the YG Ref. 6594 original?"
If I understand your question correctly, then I would say yes - the case back is correct. I do not base this on the brushing of the case back, but on the information engraved on the inside of the case back (they all look original).
The brush direction of the case back does not mean anything (even if it was the same Ref., and more so when comparing the case backs of two different References).
I think you may be reading too much into some of the details?