This diagram is taken from the parts manual on the TOC website. It is not clear but Paul’s photos help a lot. The screw is H821-100 which is described as 5 x 14 with 2 flats – and the square thing is 298.817 described as 16 x 12 x 4 with a 6.5mm hole in the middle. I don’t see the M5 nut on the diagram.
Chris