try with

000004 //S1 EXEC PGM=SORT

000005 //SYSPRINT DD SYSOUT=*

000006 //SYSOUT DD SYSOUT=*

000007 //SORTIN DD *

000008 1234567890

000009 2345678901

000010 3456789012

000011 12JKJKJKJ1

000012 23RVVRVRV1

000013 34REGRGRG1

000014 45RRRRRGG1

000015 56RGFPLFV1

000016 12JKJKJKJ2

000017 23RVVRVRV2

000018 34REGRGRG2

000019 45RRRRRGG2

000020 56RGFPLFV2

000021 12JKJKJKJ3

000022 23RVVRVRV3

000023 34REGRGRG3

000024 23RVVRVRV4

000025 34REGRGRG4

000026 45RRRRRGG4

000027 56RGFPLFV4

000028 12JKJKJKJ5

000029 23RVVRVRV5

000030 34REGRGRG5

000031 12JKJKJKJ6

000032 23RVVRVRV6

000033 12JKJKJKJ1

000034 23RVVRVRV1

000035 34REGRGRG1

000036 45RRRRRGG1

000037 56RGFPLFV1

000038 12JKJKJKJ2

000039 23RVVRVRV2

000040 34REGRGRG2

000041 45RRRRRGG2

000042 56RGFPLFV2

000043 12JKJKJKJ3

000044 23RVVRVRV3

000045 34REGRGRG3

000046 45RRRRRGG3

000047 56RGFPLFV3

000048 12JKJKJKJ4

000049 34REGRGRG6

000050 34REGRGRG9

000051 45RRRRRGG5

000052 34REGRGRG7

000053 45RRRRRGG6

000054 56RGFPLFV6

000055 12JKJKJKJ7

000056 23RVVRVRV7

000057 45RRRRRGG7

000058 56RGFPLFV7

000059 12JKJKJKJ8

000060 23RVVRVRV8

000061 34REGRGRG8

000062 45RRRRRGG8

000063 56RGFPLFV8

000064 12JKJKJKJ9

000065 56RGFPLFV5

000066 23RVVRVRV9

000067 45RRRRRGG9

000068 56RGFPLFV9

000069 //SORTOUT DD SYSOUT=*

000070 //SYSIN DD *

000071 SORT FIELDS=(1,1,CH,A)

000072 OUTREC OVERLAY=(11:SEQNUM,5,ZD,RESTART=(1,1))

000073 OUTFIL INCLUDE=(11,5,ZD,LT,3),BUILD=(1,10)

****** **************************** Bottom of Data ****************************

to get ...

********************************* TOP OF DATA **********************************

1234567890

12JKJKJKJ1

2345678901

23RVVRVRV1

3456789012

34REGRGRG1

45RRRRRGG1

45RRRRRGG2

56RGFPLFV1

56RGFPLFV2

******************************** BOTTOM OF DATA ********************************

tested also with

****** ***************************** Top of Data ******************************

- - - - - - - - - - - - - - - - - - - 71 Line(s) not Displayed

000070 //SYSIN DD *

000071 SORT FIELDS=(1,1,CH,A)

000072 OUTREC OVERLAY=(11:SEQNUM,5,ZD,RESTART=(1,1))

000073 OUTFIL INCLUDE=(11,5,ZD,LT,3),BUILD=(1,10)

****** **************************** Bottom of Data ****************************

just sorting only on the first byte( for large files might yield a more random looking sequence )

to get the same result

********************************* TOP OF DATA **********************************

1234567890

12JKJKJKJ1

2345678901

23RVVRVRV1

3456789012

34REGRGRG1

45RRRRRGG1

45RRRRRGG2

56RGFPLFV1

56RGFPLFV2

******************************** BOTTOM OF DATA ********************************