I would agree with the guys that it looks as though it could be either old bonding or browning.
What is the light/white coloured plaster around the edges of the sample pic you posted Phillip?? Could this be the original wall plaster, and the bonding/browning areas just "patches" that were repaired years ago. What i'm trying to say is,, are the majority of the wall/s plastered with the lighter coloured plaster?
Oh,, and would it be possible for you to show a picture of the other side of that piece of plaster,,, just send me the bill.