The elegantly retro, diamond-faceted decanter of I.W. Harper 15 Year Old Kentucky Straight Bourbon looks as if it has just been beamed into the 21st century from the 1920s, and with good reason: It marks the return of one of America’s most iconic Kentucky bourbons from the pre- and post-Prohibition eras. I.W. Harper was created in 1879 by brothers Isaac Wolfe (hence the “I.W.”) and Bernard Bernheim and won its first gold medal in 1885. By the early part of the 20th century it had become a top-tier brand, but a…