As I'm learning here on the farm, there are a million things that need fixing or replacing in an old house. But one great thing about an old house is that the interiors have nice, solid wood doors. For reasons of insulation (both noise and R-value) a solid wood door is always better than the cardboard-web-containing hollow-core doors common in modern homes.
However, when times were tight--think of the Great Depression, or World War II--even solid wood might not be easy to come by. Thus we see this 1940s DIY door example by an unknown thrifty craftsperson, captured by UK-based photographer Rich Sayles:
Those are solid wood strips sandwiched between two layers of thin plywood, providing the desired mass. I want to believe that these are cut-offs from some factory process, primarily because I shudder to think of someone taking the time to mill each piece to fit. And while it was undoubtedly time-consuming to assemble, I still think this is a better use for cut-offs than if they'd gone up the chimney.
It's worth noting that the UK's WWII-era motto was "Use it up, wear it out, make it do, or do without."