Fifty years ago, in 1972, the Cubs joined many other teams who had begun to switch from heavy wool uniforms to polyester. The fabric was considered more breathable and easier to wear in hot weather.
Among the things that changed, beyond the fabric, was the elimination of button-down jerseys and belts on pants. The jerseys became pullover and the pants had snaps on the front, and the top denoted by a different color. In the Cubs’ case, that was blue.
On the team’s road uniforms from 1969-71, a number had been used, on the player’s left side.