Bad RPM Signatures

Bad RPM Signatures

I'm trying to set up a repo that contains both CentOS 6.4 and 5.9
packages. The machine itself is 6.4.
I've created a GPG key and have been able to sign my 6.4 packages no
problem. When I create them for 5.9 though I get header errors. I realize
that the versions of RPM are different, so I installed 4.2 (the version
that ships with 5.9) and tried signing with that... it still doesn't work.
[root@usch1ksv01 RPMS]# /usr/local/bin/rpm --version
RPM version 4.4.2.3
[root@usch1ksv01 RPMS]# rpm --version
RPM version 4.8.0
[root@usch1ksv01 RPMS]# /usr/local/bin/rpm --checksig
ypserv-2.19-10.el5_9.1.x86_64.rpm
ypserv-2.19-10.el5_9.1.x86_64.rpm: RSA sha1 (MD5) (PGP) md5 NOT OK
(MISSING KEYS: PGP#a8228ab5)
[root@usch1ksv01 RPMS]# rpm --checksig ypserv-2.19-10.el5_9.1.x86_64.rpm
ypserv-2.19-10.el5_9.1.x86_64.rpm: rsa sha1 (md5) pgp md5 OK
I don't understand why the signatures aren't working for the older version
of RPM... I'm just doing an rpmsign --addsign , which clearly works for
the new version but not the old. I've used both versions of RPM to try
signing too.
I'm totally out of ideas, so any help would be greatly appreciated! Thanks!