We did a cd to $COBDIR/lib and ran this command:

         for file in *; do echo $file; nm $file|grep -i _tMc7901b; done

It showed that the symbol _tMc7901b is defined in the COBOL runtime system libraries, meaning it should be found as long as the environment variable LD_LIBRARY_PATH is set.

Further research proved that the script to build the program was using a callable shared object (.so file) that was created with an older version of COBOL.  Once the .so file was replaced with one created with the current version, the error no longer occurred.