It wasn't on the valve seat but on the sloped part in the intake channel, starting from the stem to the edge of the valve.
This is done on some nitro drag cars, to help enhance mixture. If the mix is 'lumpy' in a nitro engine, the engine will be in many pieces in a few seconds.
In my experience with SOHC4:
In real terms, on the street or in gasoline-fueled, carb'd engines like the SOHC4, it does nothing but weaken the already-thin valve face. You will no doubt hear much from those who tout its effectiveness, but have not dyno'd it, nor raced it against a similar, but un-similarly modified, engine, as a benchmark. And they charge a LOT to modify your valves for you...
In short, it is silly when the valves are so small...in these engines, what needs to happen most is the rapid transit of mixture between the carb and the open intake port, without too much storage time in the pocket above the valve. These are swirl-charge, directed-flow intake engines, which rely on some speed of the intake charge to prevent uneven mixture. The whirlwind velocity inside the compressing swirl-charge engines we have approaches the speed of sound at 6500+ RPM (on the 750 -- 8500+ RPM on the 350F), which stratifies the fuel mixture toward the outside of the cylinder(s), on purpose. This stratification causes a rich, fast-burn arc to ignite with the longish-duration spark, which then propagates into the leaner middle section about 1/2 the stroke distance down the bore so it can continue to push until at least 2/3 of full stroke (oversquare engines stop pushing at 1/2 stroke). This is one reason why the signature carbon patterns we see inside always appear: there is a thin layer near the valves and the edges of the chamber and pistons, with thicker build in the center, if the engine is usually ridden on the power band.
If the intake flow is disturbed with additional tumbling (as can be caused by oil buildup from leaking valve guides), the swirl pattern is broken up somewhat, and power drops off noticeably. This also happens when things like "steps" are cut into the valve face, which should be smoothed out (and the tulip lip ground off) instead.
