There are some images of the output and discussion of the \showlists trace
in the stackexchange post at
https://tex.stackexchange.com/a/734734/1090
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
