What is the deal? SBP 10"×1 3/4" backing plates.

So the drilling the 5th hole is just bs by those not knowing?
The top 2 holes look off too, which would make the drum not centered.
Looks like I have some really nice park brake cables and 10" spreader bars, e-brake levers then.

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Correct. Total and complete incorrect internet garbage nonsense. They are totally different from the 8 1/4, 8 3/4 and Dana patterns.