Chassis help - D1 & D2

[Airhead]

Hi all

Could somebody give me a clue as to what happens at the top end of d1 and d2 please? I can't see from the plans and where the book gives very precise instructions for virtually every other bit on the chassis the diagonals are covered by the statement; "Parts D1 to D6 can now be tacked in place"...

Thanks

Matt