Steve I am having a nosey around,
albeit, I only have an earlier catalogue than you, so doubtful if I can help.
A through scan shows that it is a marginal copy, from the sixth column, if I read ACSC correctly.
I presume you realise this is the 6th column of the overprint forme,
whether this can have any relavence to the stamp, I am unaware.
No idea on overprint forme size / quantity.
The dandy roll line indicate RH border, so with information on the opt forme, may well indicate your stamps plate position.
Regarding the black line through the Kangaroo (extraordinary)
look at ACSC 41g 2/- Maroon
There exists a scratch across kangaroo very similar.
I am thinking your black line may be a unwanted part of the overprint forme print, yet it seems weird that it just fitted the kangaroo.
The mark does not match the Kangaroo colour.
Also look a little above the large black line
There is also a faint line mark going across the back of the Kangaroo
This is listed as a white line in that position in ACSC 56
You need to look under magnification.