Thanx for the answer. As it seems it is no chance to find any 4.2.1 support so I'll look into installing 5.1 as it is still possible to find fixes and MLs for it.
I have an old RS/6000 7013-J50 system around here with AIX 4.2 ML 4.2.1.0. I found that latest level is 4.2.1.12 or something like that. As this AIX version is very old and out of support I...