Wow, I stumbled on this thread unexpectedly and it answers some questions i still had after owning my 320K for the last 4 years.
This is certainly a heavy machine. I'm a lifelong Singer fan but with advancing back problems, it's getting more difficult to lift this monster. I call her "the Behemoth" which fits nicely with my steampunk fiction addiction.
I was lucky to acquire her with the manual, all attachments, extending wooden plate (I don't know what it's called), and the cams. The bobbin case had already been modified, probably with a hammer and nail by the terrible look of it, but it works. I also purchased a walking presser foot which I haven't even used yet and at one point had the electrical cord replaced at the Singer service center in Seven Hills, NSW. It's still attached to the original foot pedal and plug-thing-into-the-machine but the ends of the cord were coming apart.
I've been, sadly, looking for a lighter weight portable machine. We move a lot and although I prefer the heavy monster and LOVE the aqua enamel, it's just too much weight for me.
Kudos on finding these models. I had never seen one before this. Mine says it was made in England.