Sorry for the delay, was in Alaska on vacation.
The one pictured other than a couple of small cross fold separations is actually in spectacular condition for the age and the paper is not really brittle, a little toning at some of the folds but nothing unattractive imo so it is a definite leave alone. I do have two other copies though, one of which will need backing as it's brittle and will not survive long term as is and will not be unfolded again until it is time to back it. The other I think is safe for the time being so will not touch it either but it does have a few chips and minor separations.