330 Cabin door vents... How were these supposed to work?

[Flot]

So on my 2002 330 Express, the cabin door has four black starboard "vents" inset into it on the inside.

The black starboard panels have slats cut into them for ventilation.
When you remove these, there is normal house screen behind them (this makes sense, for bugs etc)
However, there's a second layer of some sort of thin black plastic film sandwiched between. This thin plastic has almost completely deteriorated, I basically removed it with a shop vac as it was crumbling all over everything when touched.

I am not sure what exactly the intent was here? Obviously it didn't seal the vents completely, or there would be no need to put the vent in at all. But what was it supposed to do? What did it cover? Why is it there at all?

 

[everwhom]

I have an '03 and I'm pretty sure there's no plastic in mine because I often get a bit of spray into the cabin when I'm washing. I'll look at it more carefully, but wondering if there was some protective film that was never removed or something like that? Or someone put some plastic in to keep out the light?

 

[ItalianAngler]

My 02 sailfish has the same vents in the door, just house screen sandwiched in there. I remembered to check this before my first overnight so we didn't get destroyed in our sleep. I don't recall any plastic film.

 

[Flot]

Thanks guys, I hadn't actually considered that this may have been a previous owner modification - sounds like no harm in ripping this stuff out!