It seems that the base.tar.gz, put a few days ago to 
ftp://ftp.netbsd.org/pub/NetBSD/arch/pmax/binary, is broken. That's what 
I get by tar ztvf base.tar.gz on two different systems (NetBSD and AIX), 
using two independent copies of base.tar.gz:

drwxr-xr-x 0/0               0 Jul 17 19:40 1995 .
drwxr-xr-x 0/0               0 Dec 11 04:51 1994 ./altroot
drwxr-xr-x 0/0               0 Jul 13 02:30 1995 ./bin
-r-xr-xr-x 6023/0        68816 Jul 11 20:20 1995 ./bin/[
-r-xr-xr-x 3/7          178421 Jul 11 20:25 1995 ./bin/cat
-r-xr-xr-x 3/7           89952 Jul 11 20:25 1995 ./bin/chmod
-r-xr-xr-x 3/7           91152 Jul 11 20:25 1995 ./bin/cp
-r-xr-xr-x 3/7          356952 Jul 11 20:25 1995 ./bin/csh
-r-xr-xr-x 3/7          100096 Jul 11 20:26 1995 ./bin/date
-r-xr-xr-x 3/7           84384 Jul 11 20:26 1995 ./bin/dd
-r-xr-sr-x 3/5           77584 Jul 11 20:26 1995 ./bin/df
-r-xr-xr-x 3/7           64880 Jul 11 20:26 1995 ./bin/domainname
-r-xr-xr-x 3/7           56208 Jul 11 20:26 1995 ./bin/echo
-r-xr-xr-x 3/7          167968 Jul 11 20:26 1995 ./bin/ed
-r-xr-xr-x 3/7          114304 Jul 11 20:26 1995 ./bin/expr
-r-xr-xr-x 3/7           64880 Jul 11 20:26 1995 ./bin/hostname
-r-xr-xr-x 3/7           69152 Jul 11 20:26 1995 ./bin/kill
-r-xr-xr-x 3/7           64784 Jul 11 20:26 1995 ./bin/ln
-r-xr-xr-x 3/7          218272 Jul 11 20:26 1995 ./bin/ls
gtar: Skipping to next file header...

tar doesn't get any further then.

Helge