The board itself is marked Hitachi.
Now that you mention it, I think I do recall that Hitachi had announced
bubble memory devices. I've never seen one.
I can't get to the bubble memory chip because it
has a metal shield
spot-welded around it. Wrapped around 4 of the 6 sides, so to speak. I've
tried removing this sort of thing before - nigh impossible without a chisel,
and I'm not about to do that.
The shield is part of the chip packaging, not something that was added for
the cartridge. I'm surprised that the label on the top didn't have an
obvious manufacturer name or logo.