Because that's the more common & much later issued heatshield...early '80's IIRC.
Originally they were made in many different appearances,depending on your application/year,i.e. natural,brushed,polished.
Notice the 6 digit number as well,that's a tip off to it's heritage circa.In general,6 digits were a later variation of the original.
His reference of the parts book is partially blocked off,but I'll bet it has a revision date of mid 70's or early 80's.
Not technically assembly line correct for our 1968-72 vehicles,which used the 7 digit part #.
Will it fit & function for a 68-72 Bblock?..absolutley,but it's not technically correct in design & appearance for a judged car,if that's what your looking for.(Albeit most won't see it once it's on the car anyway)
Also,that design is being repopped by a many today.
Summit for 1...
http://www.summitracing.com/parts/OER-361443/