Microcode updates for current systems are downloaded via Fixdist Central, but the H50 is long out of support.
What you probably need is
this page, which lists all the microcodes and service processor updates there are. As it seems the last one (from 2004) added support for AIX 5.3 so you should indeed go with this one.
I hope this helps.
bakunin
Addendum: the download page you are looking for
is here.