Try firing up the "dsedit" application, create an entry (doesn't matter what) and see if it does not create the file for you in the expected location.
The chances are pretty good you can simply copy the old file to the new system once you figure out what the expected location is.
Do the dsedit trick and then copy the 12.5 file over the top of the v15 one you just created. It should work fine. The only difference might be that NLWNSCK may have been deprecated in favor of TCP but that is an easy fix.
Regards,
Bill