It was a swing-needle machine that came between the 206 and 319 in the United States.
It takes a 206x13 needle which are only available in 2 or 3 sizes, so I have always refrained from buying them. The 319 is interesting in its design with "typewriter keys" near the handwheel for selecting stitches, but the 206 and 306 don't have this cute little oddity to set them apart.
Some people have re-timed their machines of this type to force the machine to accept an ordinary 15x1 needle, but this causes problems elsewhere, so if you go to look at it, be sure to inspect the bobbin case and make sure that it isn't damaged from needle strikes.
All of these machines are strong and sew very well, but the lack of needle variety and the lack of distinguishing features would not entice me to put them into my collection. If someone gave me one, I would only keep it around long enough to clean it and (if feasible) restore it to good function to give to someone else.
I think $50 is a decent price if it's got a good kit of attachments and stitch disks and if the electrical components are safe and sound. It should fit in pretty much any standard Singer-made cabinet.
I have a 316G which is a German-made machine of this class - it takes a standard class 15x1 needle and it's in a treadle. I think that's the only reason I keep it - I can sew more than straight-stitching when the power is out. Changing the bobbin is rather a pain in the neck - I have to remove the treadle belt to tip the machine back because there is no slide plate. It's a bit on the clunky side - a strong machine, just not very attractive, nothing special. (until the power goes out, when it becomes my favorite machine in the house.
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