I cut my BR7 to 428mm as per cutting list on page 161
Looking at figure 4.2 on page 34 the overall dimensions given to the outsides of BR8 & BR9 is given as 478mm
Deduct 50mm for those 2 tubes 478mm - 50mm = 428mm length for tube BR7
The drawing is correct as far as I know, I do not remember any amendments to this part of the chassis.
No idea as to where you get the 409.92mm from !!??( 410mm in the real world )
