UPDATE to bunker tours (noted on p. 134 of guidebook):
Several groups near Ravenna are restoring the concrete bunkers that line the coast in this area. Members give tours in the summer, usually in Italian. You can see their activities on social media sites (as of March 2020) by searching with these names:
Bunker Tour Cesenatico
Bunker Tour Galla Placidia
Bunker Tour Punta Marina Terme
One of their goals is to make the bunkers an open-air museum. See below photos of selected fortifications in Punta Marina Terme, a village close to Ravenna.
Click on the photos to enlarge them. Scattate le foto per ingrandirle.