Flash is traditionally number 4 after the famous trilogy. Barry Allen is the Flash most people are familiar with and he is patterned after the Gods Hermes/Mercury. Since they are seen as the messengers of the Gods it seem appropriate to have this be a letterbox. The stamp inside is a tribute to Barry Allen being a CSI. Collecting fingerprints was the best possibility.
Please replace as found.
Be sure to collect the Batman clue.
Oneel pbhyq irel rnfvyl cnff guebhtu gb gur pyrnevat.