Plastic behind wood cladding

[AlBob]

We had external wooden cladding removed from between ground and first floor in our uk 60s build and this is what was revealed. It's now been clad back over in hardie plank but I wanted to know if people knew what the plastic was protecting and what would have been directly behind it e.g. ends of the floor boards (or would they sit above/below or behind something else), a beam or cinder blocks or something else?

[Big Jimbo]

Before it was clad over in Hardi plank did you provide a vented cavity to carry the boards ?

[AlBob]

Yes, they built out with batons. Anyone familiar with 1960s crosswall constructions able to advise on the plastic and where the floorboards are in relation to it?