Lath for plaster, following wood lath era!
There is no way that would be treated like drywall. Too many joints!
Exactly.
It's gypsum board, usually 18" or 24" wide. it was used as a backer board in the 50's and 60's while the construction industry was transitioning away from wooden lath and before taped and spackled sheetrock became the standard. It normally had two coats of plaster applied, a brown coat and the finish white coat. It is usually about an inch thick but varies quite a bit.
I deal with it all the time in remodels. The metal lath used on inside corners is a pain in the butt.