Assuming the original files were FB/80, and with the identifying character added at the end of each record, they are now FB/81, here is an example:
//STEP1 EXEC PGM=SORT
//SYSOUT DD SYSOUT=*
//SORTIN DD DISP=SHR,DSN=FILE1
// DD DISP=SHR,DSN=FILE2
// DD DISP=SHR,DSN=FILE3
// DD DISP=SHR,DSN=FILE4
// DD DISP=SHR,DSN=FILE5
// DD DISP=SHR,DSN=FILE6
// DD DISP=SHR,DSN=FILE7
// DD DISP=SHR,DSN=FILE8
// DD DISP=SHR,DSN=FILE9
// DD DISP=SHR,DSN=FILE10
//SORTOUT DD DSN=OUTPUT.FILE,DISP=...
//SYSIN DD *
SORT FIELDS=(81,1,CH,A)
OUTFIL FILES=OUT,SECTIONS=(81,1,
TRAILER3=(COUNT=(EDIT=(TTTTTTTTTTT)),1X,81,1)),
REMOVECC,NODETAIL
/*
The output produced would be as follows:
0000000010 A
0000000020 B
0000000030 C