manicmike

The bobbin cases (which I think they're different for each model) can be modified so these machines can safely take the normal domestic needle (15x1). I had an OSMG do it so I could use any needle on my 320K2. Schmetz only make size 12 and 14 in the 206x13 now. Using the slightly longer 15x1 is also quieter because of the smaller angle the needle makes with the fabric. Use of 15x1 in a machine with unmodified bobbin case will cause the needle to hit the case during zig-zag stitching.

A few things I know about this series:
The 320K2 is the ultimate in this series and is a free arm version of the 319. A brown version of the 320K was made (320K23, I think), and it's a free arm 306. Both machines lack a droppable feed dog.
At least one model of 306 has no ability to use cams (306K22).
Only the 206 is cast iron. The one I have (light industrial model, 206K10) is enormously heavy and also doesn't do cams.
From what I've seen and heard (Great White has also mentioned this) it's common for the hook timing to be out. Either because some person has decided this is the best way to allow 15x1 needles to be used or because they've been used to sew heavy canvas. When the timing isn't factory standard they don't stitch well at all. Luckily it's easy to change them back.

I have a soft spot for them because my first sewing machine since starting this hobby was a 319K. Beautiful machine and could do practically anything. They do embroidery really nicely too.

As nice as they are, none of them do as good a straight stitch as a good old bulletproof 201.

Here's my 320K2: