"The
vent is supposed to be the scored mark on the top and the plug at
the bottom".
Most older designs used the scored topside as a backup.
A few vendors used it as the sole venting means. But most just have the bottomside
vent plug.
Between 2001 and 2003 the market
was flooded with bad caps. The problem had serious impact
on both MoBo and PSU vendors, among others. A lot of
products containing bad or marginal caps are still out there.
At the time, it was written up in
the Electrical Engineering Times and other trade rags.
Rather than go digging for those old, dry technical articles,
have a look at PCmag's
Hushed-Up Disaster Dept.
To better help identify bad caps,
be they on the MoBo or in a PSU, see the following:
Bad MoBo Caps
Bad System Board Capacitors!
How to identify bad capacitors
The above have plenty of good close-up photos.
"Looks
like I have 5 of them dead. 2 bulging, 2 leaking a little out the
top and 1 where the bottom plug has been forced out".
Sounds as though your PSU was long overdue for replacement.