Here's the deal - you have an 04+ Impreza. You want to use some new struts or strut inserts, but the problem is all the ones on the market are for the 02 or 03 Impreza.
Well, there's a couple of things you can do. 1) Get 02 or 03 rear top hats (that's the expensive solution) or 2) you could do what's called "the washer mod." Basically, it's a stack of washer put on top of the strut to get the spacing right so you can use the tophats you already have.
Problem with #2 is you're left guessing thicknesses, mixing and matching washers, worrying about clunking, taking off and putting back on struts, complaining over and over again every time you have to do this. In other words - it can be a real pain.
That's where Scooby921 came in. He had this great idea - why not just machine the proper spacer? So he did. And people rejoiced. And then they got popular. And then he came to us. And we rejoiced. And we bought the design. And now we have them. Life is good.
Oh, and here's the details - these little critters are made from 4150 steel, and have a nice pretty black oxide coating on them.
How this works:
You get 2 spacers. You use them on the back struts only. The fronts bolt right up so you don't need them up there.