Air suspension on '73 dart sport?

Hi all,

Below the rear licence plate of my '73 Dart Sport there is a sort of valve (like on the tires). Behind that, tubes run towards the rear shocks (I think). The rear end of the car seems to be raised quite a lot, could it be that this is some sort of air shock system?

And if it is, how could I lower it a bit, what tool should I use and towards how many PSI?

I have included some pictures, hope they give an image of what I mean.

Thanks for thinking along!