Prusa I3 Ver4 P3Steel --top Z bracket question

Hi I have just purchased a ver 4 frame. 200x300 print bed.
The question is
1. Why do the 200x300 versions have the top Z mount brackets looking like an L shape. ---unlike the regular 200x200 P3Steel versions? There is not a mount position (support) for the 5mm lead screw. I have the full metal Z axis but am not sure whether I will be using it. I would prefer to go for an 8mm trapezoidal lead screw.
2. Could I use the standard type top z Brackets? Is there a file I could download to laser cut new brackets?

The other photo is of a bar 3mm thick x10mmx25. What is this bar for / location.
All help would be appreciated

Thanks
Paul