[Carbidetooth]

This sounds like it may be an early iteration of SIPS (structural insulated panels) construction. At one time, floors were considered a good candidate for SIPS, in addition to walls and roofs. As far as I'm aware the method has been largely abandoned for floors. I can't imagine what you describe as being very stiff, but who knows.

I don't know if it will provide the deflection info you're looking for but might be interesting if you want to learn more about structure.

1. Are the plywood layers bonded to the foam?
2. What does the foam look like? EPS looks kinda like a styrofoam cup. Polyiso foam boards are usually kinda creamy in color and denser than EPS. XPS foam boards are typically blue or pink, but not all that common with SIPS, as far as I know.