/etc/shadow Locked accounts

[redhat at buglecreek.com]

On a RedHat ES 4 system I was checking for null passwords in /etc/shadow
and came across a "x" in the encrypted password field (2nd field) for  a
few accounts.  I have seen "!" "!!" and "*" for locked accounts but have
not come across a "x". The x does seem to lock the accounts however,
which is what I want.  Anyone know what program may have placed the x in
the field? The "usermod -L" and "passwd -l" commands does not seem to. 
Also, does it have the same effect as "!"?  I was just curious if anyone
else has seen this.

Just to clarify, I am not referring to the /etc/passwd file.

Thanks