panton stress crack  

Just to satisfy...
my curiosity. Is it one of the "old" polyester & fiberglass chairs or a new PP Vitra version.

it could be
that you had a really vintage panton! i have heard once that the very first models did crack in that place and that later models had reinforcements brought in, that was the reason why panton was ok with the new vitra version (which do not have thes inforcements anymore)

chair was made in 74 and...
chair was made in 74 and does have small webbing on back side of knee