This from Mike Summa:

Hello,
This is a Chicago ladder truck, taken around 1975, at a parade in Posen.  Yes, Posen.  I didn’t know that Chicago would stray that far to participate in a suburban parade.  It was a surprise to see it with all the south-suburban trucks.  Please feel free to add any details/information.
Happy Fourth of July,
–Mike Summa
vintage Chicago FD ladder truck 51

Mike Summa photo