The reason that particular trunk weather strip on EBay is so expensive, is because it has the white string in it.
To my knowledge this was what may have been installed at build. I have removed one of this type on a 69 L78 Camaro.
Alot of the later N.O.S. replacemrnt seals are not near as expensive, and they do not have the string in them.
The string may have been in there to keep from stretching it to much on installation
Mike