There are some images of the output and discussion of the \showlists trace in the stackexchange post at

but this plain luatex file

\tracingonline1
\showboxdepth10

\immediate\write500{\meaning\Arrowvert}

$\Arrowvert + \Big\Arrowvert\showlists $

\expandafter\show\the\textfont3


\bye

produces the expected || character with pdftex but a { character with luatex even though both claim to be using the same extension font (cmex10)

David