Don't know what the deal is on these 2 Barbie's I have. They've been stored and are as old as all my other ones and these have major discoloring on their legs. I can only assume it was a defect in the original plastic that showed up over time. Originally they were just fine. Quite disappointed in Mattel. :(
Wow i have never seen anything like it!1 but yes it looks like a "plastic" manufacturing defect.
Maybe someone here has seen this before..Yours can't be the only one. Put 'defective plastic in vintage Matel Barbies' into the search bar for Google maybe something will come up!