I got the file names from the following document on this board:
https://www.ubbcentral.com/support/docs/ubbthreads/UBBclassic_to_UBBthreads_tips.html

They are mentioned in Step 4 under the heading, "Instructions for Performing the Actual Import"

I have searched high and low for these files, but got nada.

I did perform a successful test upgrade from classic to threads 7.5.6p2 using "classic_import.php", but that's not what we really need.