Not the proprietor, but happy to chime in with whatever I know.
The material is probably FR4 (Fire Retardant). That's probably all the information you will ever get. If UL is asking you to trace the PCB material to the original manufacturer, you may as well give up now. Unless the Arduino folks (who presumably are the only suppliers of these boards to Mouser) can trace every batch of etched PCB's they ordered back to the source (most likely in Asia), who can then tell them which sub-contractor they used to make every particular batch of boards, who can then certify the board material, you'll just never know.
If the Arduino folks step in and say "yes, every Arduino ever made and shipped to Mouser was made on FR4" then you may be able to quote that.
UL is funny business. Good luck.
The DIN Rail Mount
kit for Arduino: quickly attach your Arduino to standard DIN rail