This is an "event" cover commemorating the anniversary of an "event." In that sense it is not unusual at all. Here's another one produced by the Chicago Philatelic Society in my collection:

If there is an "unusual" factor here it is the event itself. Based on these two Chicago Philatelic Society covers they created event covers for events largely ignored otherwise by the philatelic community. They are still around, as CHICAGOPEX, but most of these kinds of covers seem to date from the late 1940's to early 1960's. Here are some more from an
ebay listing:


Basil