Take Your Brass and Shove It
This is what I got instead:
The top of the access panel is held in place by these two nails (there are driven into the jamb on the "inside" of the weight pocket).
And last of all, the top of the access panel. Someone spent a lot time notching this out, glad it wasn't me. You slip this end in first until the nails lock the top of the panel in place. Then you nail the bottom into place.
Functional? Well, mostly I guess. It’s not awe inspiring though. If I were to guess, it looks like the window jambs were built and the access panels were added as an after-thought before they were installed.
Another oddity is the three smaller windows added in the 1920’s or so have no access panels at all. Yes, I know they should have some, but they don’t. Very annoying.