E70-15 white wall

[X66 714]

In March of 1976 I became the 9th owner of a 1968 Z/28. Car had 43k on the speedo & always had an E70-15 tire in the trunk on the original DF wheel. A few years back I replace all the tires with new E70-15 Goodyears. I put that tire away because I've always claimed it to be my original spare. After doing some research here I discover it is not the original tire. Now I would like to possibly figure out where it might of come from. It's a thin white wall. Says Custom Wide Tread & below that it says polyglas. It has outlined letters, not raised white letters. The date code is TS01161 which would be April 1968. My car left L.A. March 15 1968.
What car would this tire have come on? Thank you, Joe