Picked this stuff up at a hobby shop in LA when I was there in April - it's called micro mesh and it's a very fine brass mesh. I've had in mind to do some detailing of the basement windows on my Walther's American Hardware Supply kit, which I started quite some time ago and then kind of set aside. I've certainly walked by many an old factory that had this mesh over the basement windows and I always wanted to model it. Here are the first results. Actually this will look a lot better once I get some weeds in place and the back put on. What you're looking at is the windows in the lower right of the photo.