I think bunkers should have their own category. They're definitely different enough.
I'll add that there are lots of places like Fort Warden here in Washington State that I'd love to add the bunkers into the different waymark category. Also as long as the alternate term relates to blockhouses (using Wikipedia as a guide) there's no issue with listing them. I don't mean a plaque to be a requirement but there should be some local naming convention for the structure that relates to a blockhouse.