Pollen filter side only.
Now to put the cat amongst the pigeons
I do not have the "spy hole" and have in 21 years of ownership not needed one.
The secret? Jules' plastic insert on the drivers side of the plenum AND blocks of large pore acquarium filter material on each end, blocking any access to the plenum chamber. Very minor debris through the plenum chamber covers.
Although I am surrounded by many trees including Scots Pines with their needles especially at this time of year, an annual vacuum cleaner suck is all I have ever done.
