I just looked in the Henry J parts book. Quadromaniac, I would say yours is original and upon further review, it appears Vagabond Russ' is also original. My memory is not what it was. There is a part number listed in section 30.611 of the '51 Henry J parts book which calls out "Panel, Center (used less deck lid) 210986" and the '52-'53 supplement section also lists "Panel, Center Dinosaur grained cardboard (used less deck lid) 211544" and "Panel, Center Ribbed grain cardboard (used less deck lid) 211545." Sounds like Quadromaniac's is the former and the additional two listed in the supplement are listed for K533 only.