I just saw this picture of wire.
Pirelli Cable-X – 500 MCM Type THHN or THWN-2 600V Gasoline & Oil Resistant II AWM (UL)
Wait a minute.
It's routine common THHN, like you might use in a 250A service or something. But despite all this detail it seems to have forgotten to state whether it is copper or aluminum. And tell you the truth, I've never stopped to notice if any of my other THHN also fail to mention.
Best Answer
The UL publishes a guide on wire and cable markings:
UL Marking and Application Guide https://www.ul.com/wp-content/uploads/2014/04/WC_MG.pdf
The "default" is indeed copper.
However, the AWM in the marking would lead me to believe this may be something other than building wire per NEC: