Part #12 is shown on page 9 of the plan.
They are the two (top and bottom) horizontal wedge supports.
I just verified the measure measurement in CAD. The two boards are 31 1/8″ at their longest point with both ends mitered at 45 degrees as clearly illustrated.
(I did find the error in the CHBD and have corrected part #12’s length from 38.125″ to 31.125″)
Part #52 is floor panel B2 and is shown on page 14 of the plans.
This part should be manually measured and custom fit as shown on page 14. It is not resizable.
I think Lloyd also mentioned the lack of instructions for the foot rest top.
It’s the last thing you do and should be measured by hand.
I think most folks can handle that.
Remember, the plans are a guide…not law.
Thanks for the feedback!