This year’s 50th anniversary of Hot Wheels is making yet another victory lap in its yearlong celebration. Toymaker Mattel’s tribute to its free-wheeling toy cars began April 28 with a 14-city tour to seek out crazy cars for a new series. And now the U.S. Postal Service is honoring the diecast cars with a block of 20 Forever stamps showcasing some of the most outrageous designs from Hot Wheels history. To be sure the stamps...