306W bobbin case....... I seem to recall there being two cases .. the 319 is I recall almost all got the "open" style and the 306 a closed style ( later ones maybe getting the open?) in any case the one that came with my 306 is a closed one ( full cup with just a cutout for the needle) and I have never dealt with any of the others. mine works ...... if you want pictures I can try to help, but will not be back to my machinefor a couple weeks. Have you seen my video of the 306 in a treadle stand? it works rather well.
Dan
http://www.quiltingboard.com/vintage...o-t185409.html
thats a black 306 in an old ( not fully adjusted and clunky) 27 treadle... the treadle is made in 1900 112 years old... the machine is mid 50's