Try Brian Ewing @ Tuscany Village if you don't get a timely response on this forum. Since you mentioned 75 years old and 'plaster', he should be able to guide you. He frequents other forums and has one of his own. He's a Master Plasterer (I guess you'd say). W/O getting behind the wallpaper to see what's there, I wouldn't know if someone, over time, put up drywall then put up wallpaper. I would suspect not but you never know. Perhaps before you write to Brian, you could 'break' a less-than-conspicious section of a wall to see what's there. Plaster is easy to replace in this particular instance AND since you're doing it in a less-than-conspicious place, any imperfections will be out of direct eye sight.