CHICAGO — One day this summer, in the midst of the pandemic, Eric Wolforth took a walk to the store to buy milk and diapers. He followed his usual route, west along Catalpa Avenue, then a right turn into an alley.A few doors up he spotted piles of junk around a cluster of garbage bins. Knowing that Chicago alleys often contain discarded treasure — he once salvaged a fancy stroller — he headed toward the mess.On top of one garbage bin sat a small box. Poking from the top were red and blue stripes he recognized as the marks of old airmail envelopes. He pulled out several letters.In the upper lef…