Before 1991, What ingredient was different between the plain and peanut M&M's?
Before 1991 both plain and Peanut M&M's contained peanuts. The peanuts were ground and added to the chocolate. Thus the ingredient was the same on both packages. The area in the list was different because of the amount of peanuts. When they first came out they were used in military K-Rations because the peanuts had protein in them. The old slogan "Melts in your mouth, not in your hands!" was related to this partial fact of the candy's not making a big mess for the soldiers.
The recipe was changed in 1991 due to many people having Peanut allergies. The Plain M&M's are made separately, and the packaged states "Peanuts may be present" just in case peanut dust somehow gets to the Plain M&M area. Should not happen, but just in case.