I've had this old Champion spark plug for near 40 years and I've always been curious to find out what engines would have used it. I did find out it was a racing plug but that's all. It's a Champion R-17. The 7 didn't come out in the photo. Thanks!
I found one use, for a Poulan chainsaw. The large, older logging saws, such as the Pioneer and McCulloch, also used this kind of short plug. I have a bunch of similar plugs. Bob