What do you think this odd layer under wall paper may be?
It is likely wall liner. It is a heavy plain paper that is applied prior to the outer wall paper. You could think of it as a thick coat of primer that will hide a multitude of problems. I have seen it applied over dark patterned paper so a lighter pattern can be applied with stripping the walls. I have also seen it used over the scratch coat or when the wall has a lot of repairs to give the wall a smoother appearance.