Originally Posted by
Retro Grouch
If it's exactly opposite the valve hole that would make me think it's the rim joint although I've never seen one that was angled like that [emphasis added] before.
You never saw it because you cannot make a rim with an angled joint. When built the rim is under intense compression, running along the circumference. A joint hat isn't exactly radial would invite slippage of one side under the other causing the rim to buckle.