Hi Mike
Have a closer look at the book
The material thickness is noted below the part number.
For example, page 178
RU1
3 plate
Or page 179
RU7
10 plate
A little confusing as the mm has been omited
That said, the suspension & shock brackets, do not have a material thickness on the drawing & they are 3mm ( Tis mentioned in the text though )
I hope that helps a little