steering box spline size??

[burlapen]

Trying to swap an aftermarket column into my 63 C10. It has a power steering box from a 1971 or 72 that I put on years ago. The new column is double D style so I ordered a D to 3/4"-36 adapter thinking that would be the one. When I popped the old rag joint off, the steering box spline looked different than I had thought. It has too flat sides so it is tough to count the splines but it measures around .800 and the adapter definitely doesn't fit. Any ideas what I'm looking at? Thanks