It didnt look too bad.

BUT... I noticed that the dirt that was on it seemed to concentrate on one side only.
As if the airflow had been somehow disturbed.

And low nd behold as I looked around the airbox ( which was more dirty than my liking luckily only on the one, the "right", side) I noticed that the mating surface for the airfilter, where the airfilters rubber gasket mates to the airbox was real dirty.

And in caked on dust!

there is no way the airfilter could properly seal under those conditions I thought.
So I meticulously cleaned all the mating surfaces and hopefully my new airfilter will seal well to the airbox.

But its nice feeling now to have a nice clean airfilter in there and a cleaner airbox..

That's typical for the air filter to get dirty just on the one side; it's the way the air is directed. Some have tried the "air box mod" where they cut away the plastic duct extension inside the box, but I've never heard of any real benefits afterwards and would advise against it. The air will flow to the point of least resistance in the air filter. You can always turn the air filter 180 degrees, but I really don't think that makes any difference. One of the biggest pains with the design of the factory air box is trying to reassemble it, getting the three tabs fully engaged into the slots of the bottom air box. After struggling with this a few times, I realized it's a lot easier to remove the air intake tube between the upper box and the throttle body and then install the top of the box. Once that is on and the clips engaged, then I just reinstall the intake duct. It's a little more disassembly/assembly, but I find it works a lot better and doesn't really take any more time because it saves the time I would otherwise be trying to get the top of the air box on properly,

I carefully cleanied the mating surfaces and made sure the new airlifters fit properly.
I agree about the clips in the back.
I double and triple checked them on both the R51 and the N50 to make sure they were "in".