@salmonmac forgive me for butting in, I think there is a mistake here.
The rib 3 cast off 1 would make 4sts and a gap.
4, gap, 11, gap, 11, gap, 11, gap, 11, gap, 1
49 stitches and 5 cast off gaps
However, at this moment I don’t see how the next row lines up as I thought the cast on would align over the cast off creating the button hole but it looks to me like the cast on is a stitch out of alignment with the cast off. Now that could be my inexperience…or it could be that they don’t align.
The second row presents on the left needle as
1, gap, 11, gap, 11, gap, 11, gap, 11, gap, 4
My logic says it would be worked as
Rib 1, cast on 1, rib 11, Co 1, rib 11, Co 1, rib 11, Co 1, rib 11, Co 1, rib 4
To make the cast on stitches line up with the gap of the cast off stitches.
Perhaps this is the reason @LBFL is struggling iith working this section due to the cast off and cast on not aligning?