The photos helped. They showed me more views of the unit than I had found in my research.
It does appear to have a built-in amplifier. The socket on the front is the headphone out socket (as you have discovered). The headphone jacks were big back then. The sockets on the back are known as phono sockets but - and this is an important thing to understand, the two sockets labelled "PHONO" (left and right) are for a turntable (vinyl record player). (I know, confusing eh?) If you plug the iPod into those sockets you may find that the sound suffers from distortion as well as producing an unnatural sound.
If so, you should find better results by plugging the two phono plugs from the cable into the TAPE sockets on the back of the unit instead. The volume will be lower but more controllable. Oh - and by the way, the two phono plugs on the cable should be different colours, usually red and black. The red plug is the right channel (so should be plugged into the TAPE - RIGHT socket) and the black plug is left channel and is plugged into the TAPE - LEFT socket. It will not do any harm if they are plugged in the other way round.
To recap: the sockets on the back of the unit are known as phono sockets (that's the type of socket/plug) and one plugs phono plugs into them. The sockets labelled PHONO (that's its purpose) are phono sockets but specifically for a turntable that plays vinyl records. There's more to this than I need to write here, so just accept that one would not usually plug anything different into that input. The results would not be good.
It's an old unit, possibly from 1969. By the time I bought my first stereo system (a few years later), phono sockets were no longer used for the speakers, possibly to avoid confusion about what to plug in where.