One per plug?
I wonder what's inside it? Probably just a short air gap surrounded by insulating material.
Steve, I have several different brands of those type of items. I think it goes in the coil only. I remember they were still being sold at our local parts store back in the 70s-80s era. Like Mark, I think they just have an "air gap" inside the case. An air gap will intensify the spark and "sometimes" make a "missing" plug fire. They do work to a certain extent, but probably should be included with the other "snake oil" type of parts ...
Oh wow, I have 500' of shelving in my garage and may have to go searching thru them to see if I have one so my wife will not think it's all junk!!
Always at the Guernsey County Fair a fellow with a '50 convertible Cadillac would demonstrate these things. Had a big tach. you could watch, hand throttle, quite a rig.
Engine would pick up a few RPMs when running on this.