Next time you pull them off to have a look see, take a look at the bottom side of the shaft, and particularly at the bolt hole in the shaft where the oil comes up from the cam thru the head to oil the rockers, and see if that hole is larger than the other hold down bolt holes in the shaft. I dug out some old factory rocker arm shafts today to see if there was a difference in that hole size, and there was. 2nd hold down bolt hole from the end when you bolt them down correctly. Hold the shaft with the end that has the notch in your left hand and look at the 2nd bolt hole in to see if it is bigger. Interestingly, I looked at a set of rocker arm shafts that came off of a set of 302 heads, and the 2nd bolt hole from each end is bigger to let oil get into the shafts and feed the rockers. Those 302 shafts have no notches in them, and can't be put on wrong like the older ones. Check that on your 273 rocker shafts and also the aftermarket ones that keep screwing up on you. I bet the 273 shafts have the bigger hole to let the oil into the shaft and the other ones may not.