Yep, you nailed it. I called it a 1x6 for simplicity, but yes, it does measure 4-5/8. I figured it was a 1x5 nominal (if there is such a thing) and wondered why they used such an odd dimension. I get it now. The board is just ripped to fit the frame.

I think you're saying the sides of the entry are framed with 2x4 wall studs, and there's a single 2x6 above the entry, with the 2" edge centered on the 4" faces of the studs.

Omgosh, and that's why on a separate project hanging curtain rods, I was going through drywall, finding open space behind it, but then kept hitting solid wood no matter where I drilled. I wasn't hitting vertical studs, I was hitting the horizontal window header. Which is probably a 1/2 inch back from the drywall... Thank you.