Beaver board walls?

[Timfoxep]

My son has an early 1900s farm house he is slowly renovating. He is planning to take out a walk between the kitchen and dining room. He started a smallish hole in the sheet-rock to see what was behind it (there is a chimney in there somewhere) and immediately ran into a soft wood (feels like cork board) layer against which the sheet-rock was installed. Someone suggested it might be beaverboard. The interesting bit is it has a pattern in a bluish grey on a light beige background. The pattern looks to be a heart with some letters/figures inside, as well as what appears to be the date 1788 (see the attached picture - it’s about a foot square).
Any ideas what he has lurking in his walks?