New steering coupler doesn't fit!

Believe me it was quite confusing to me as well. What I do know is that some replacement couplers have the master spline while others do not. Also for whatever reason my manual steering box doesn't appear to have a master spline on it as well. I do know that the box was rebuilt. I'll have to look through my old receipts and see if I can find the company that did it and see if they changed the shaft in the box? I realized that if something is screwed up it usually happens to me.