New steering coupler doesn't fit!

I think I noticed something that seems strange to me. I have purchased two new couplers in the last several weeks. Both look the same dimension wise but have different threading associated with the notch machined into the bottom of the coupler. One has a blank spot right near where the notch is machined the other does not and has a continuous thread pattern inside the bottom. I also noticed that the old beat up one I took off also has a notch machined in it but a continuous thread pattern. So I'm thinking my steering shaft coming off of the steering box probably doesn't have a master spline marking point? Am I correct in assuming the only way to check this is to jack up the front of the car to get underneath and try to see the underside of the shaft? My manual steering box was rebuilt several years ago. Could they of replaced the shaft with one with no master spline marking point? Somewhat confused!

Carl