From the brief bit of head scratching that I done.
It would appear that it is the intersection point at the bottom of the front frame tube & the Two upright tubes that attach to it.
I can't name the parts because I do not have rhe book to hand.
If I were to build another chassis, I would ensure that the front frame was made to suit the brackets.
With the chassis fully welded, altering the front frame was not an option.
