The heels are resoled/reinforced with green material, likely rubber or a rubber product.

In a thumbnail photo on the previous page, look at the heel tab and note the elastic that has been sewn into the shoe. This is an accomodation for the fact that the heel band was too tight for many runners; I got blisters on both heels the first day I wore them.

It was not uncommon to see this change made on early Cortez shoes.

The change to my shoes was made at Jim the Shoe Doctor, an aptly named shoe repair business in the same building as the first Athletic Department in Eugene.

See Shoes #12 for the softened heel cup that Nike designed to address this issue.

Geoff Hollister, in his book Out of Nowhere, also refers to the need to relieve the pressure of the band that kept the heel in the shoe. It was too tight. 

"In the Cortez, the top line crossed the bone, and after two months in the shoes, I was getting a sharp, concentrated pain in the middle of my heel. I slit the back of the Cortez topline with a razor blade to relieve the stress. I didn't complain about it and kept my training a priority." 

(Hollister spells 'topline' and 'top line' two different ways.)