As recounted in the previous post, I had to sister the cleat on bulkhead 2 to compensate for it being attached 1/2" too high.
The floorboards are 3/4" thick, so I would have to cut a 1/2" rebate in the face of the forward end of the affected boards for them to sit snugly against the bulkhead.
I decided to use the router and a 1" two flute cutter, with the fence attached.
I also made a jig to hold the boards securely in the correct position, and to provide a long and even face for the router fence to move against.
Here is the set up with a board in place before the rebate has been cut.
And here is a board with the rebate cut, nice and clean.
Eight boards in total butt against the bulkhead, so I selected eight lengths of Ash and chose which face to be uppermost, and cut the rebates.
It has been a lot of extra work to remedy the unexplained error, but I am confident that the outcome will be satisfactory.