What are these ports on a 20th anniversary Mac?
Wow, that's a find!

Actually, there are more than six ports on the rear, there's also a cover you can open to reveal a whole heap of other ports (e.g. Serial ports, ADB port, etc).

However, to explain the ports in your question:


Port 3 is a SCSI port
Port 4 is a TV Tuner port
Port 5 is a FM radio port
the port on the left side you refer to is a PCI slot (Note there's actually a comm slot above this as well).


Below are some images I've taken from the Twentieth Anniversary Mac Service Source:







IMPORTANT NOTE

This model actually has a switch to select the appropriate voltage for your country (see below), so make sure you have the correct voltage selected for your location.



If your location is not listed, then select whichever is appropriate (e.g. if you're in Australia then select 230v).

[EDIT]

Finally, as per IconDaemon's comment below, you can access the service documentation directly for yourself.