I will take another stab at this. The fire management officer umbrella Hopewell Cultural is Indiana Dunes. 166.375 might be the frequency for that unit. I seems interesting that Hopewell does not have a radio system as even though the acreage of the park is low, it is in 7 different and widely spaced parcels. I would bet, based on the verbiage in the burn plan, there are no mobiles or portables assigned to Hopewell. The burn boss and the remainder of the 12 people assigned to this prescribed fire must be bringing radios from Indiana Dunes. Some of those 12 might be from other places in that NPS Region. Never the less, they are probably using Indiana Dunes radio frequency.
This is very spotty information and reminds me of reading U.S. Fish and Wildlife Service burn plans. Some National Wildlife Refuges don't have radio systems either, due to small size and distance from other refuges. The burn plans are often very vague about radio systems, frequencies, if people on the ground can reach a dispatcher by radio or phone and other similar details.
I may have a book at home that might show frequency assignments for NPS units in the midwest, however it is dated 1996 and is the information for small park units east of the 100th meridian is usually sketchy. The notebook is buried right now anyway. 6 weeks of remodeling and I can't get my office organized because one door has been put in backward and I can't mount one bookcase to the wall to make room for tearing it out, which hasn't been done yet, so most of my books are sitting in big stacks. Arrrgh!