The reason that none of the aftermarket shafts are grooved is the fact that the rockers themselves have an oiling groove machined 360* inside them which lines up with the oil hole in the shafts. this provides oil to the whole inner bore of the rocker.
This is not the case with the 273 shafts the entire inner bore of the rocker is smooth and it sits tight over the oil hole in the shaft and very little oil can get out of the shaft. The stamped rockers have a small groove built into them that lines up with the oil hole in the shaft so they get plenty of oil.