Nancy per the Vintage Singer Group The 306K and the 306W do not always use the same bobbin case. The 306W use
bobbin case # 105032 -- the open front one.
The 306K and the 319 use #173058, the closed front bobbin case. Though, some machines can interchange...Singer started switching out the hook assemblies of the models some time during manufacturing. Did you get a 306K or W?