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Mike Davision did in his heads, I used them on the Hydro engine. Also Kong heads had them at the top of the head between the valves. Not the best place. The combustion process starts at the plug, and travels outward toward the farthest part of the chamber. AS this takes place pressure is built up in the cylinder pushing the piston down, However, as the pressure builds, it can reach a point where the unburnt Fuel mixture will detonate. This id the Knock we call spark knock. Now all combustion chambers have an octaine number like gasoline. THe factory will let you know what quality gas you need to use in your engine.
If in your flathead engine you have a tight piston to head clearance verylittle Af will be over the piston when the spark occurs, and it will be traveling at a high rate of speed toward the transfer area. Bringing the plug into the chamber we burn more of the AF before etion cah occurs by shorting the flame front, especially with a power tip plug. This improves fuel economy as well as increasing power from a lower grade of gas.
Back in the 29's an englishman wrote a "white paper" on combustion chamber design and created the term "Octane". I forget his name, but his work started the evolution of combustion chambers we have today.
Now I have to fix all the red lines under my misspelled words. Eye sight is getting worse.