What if a user or group name is a string of digits?

root>useradd 1991
root>mkdir /home/1991
root>chown 1991.1991 /home/1991/
root>passwd 1991
Enter new UNIX password: 
Retype new UNIX password: 
passwd: password updated successfully
root>login 1991
Password: 

The programs included with the Debian GNU/Linux system are free software;
the exact distribution terms for each program are described in the
individual files in /usr/share/doc/*/copyright.

Debian GNU/Linux comes with ABSOLUTELY NO WARRANTY, to the extent
permitted by applicable law.
$ 
root>deluser 1991
Removing user `1991' ...
Warning: group `1991' has no more members.
Done.
root>


root>ls
Desktop
root>cd /home/
root>ls
1991  jeffrin
root>rm -r 1991/
root>cd
root>



root>useradd 471
root>mkdir /home/471
root>chown 471.471  /home/471/
root>ls -l /home/
total 8
drwxr-xr-x  2 471     471     4096 Apr 15 17:57 471
drwxr-xr-x 24 jeffrin jeffrin 4096 Apr 15 17:25 jeffrin
root>chown +471:+471 /home/471/
root>ls -l /home/
total 8
drwxr-xr-x  2     471     471 4096 Apr 15 17:57 471
drwxr-xr-x 24 jeffrin jeffrin 4096 Apr 15 17:25 jeffrin
root>chown +471 /home/471/
root>ls -l /home/
total 8
drwxr-xr-x  2     471     471 4096 Apr 15 17:57 471
drwxr-xr-x 24 jeffrin jeffrin 4096 Apr 15 17:25 jeffrin
root>

https://www.gnu.org/software/coreutils/manual/coreutils.html#Disambiguating-names-and-IDs