Jon,
I have some of those 'looks like, but is too big to be a W&W8' bobbins too. Mine are stamped with a tiny '4' on one side along the outer rim. Those things are thick. Very curious. I will go try one on my IM, but that machine takes the same bobbins as the IF (I have only 1 between the two machines).
Beautiful thimbles!
Suzanne