This is the only 3 brush powerhouse picture I have in Photobucket. You can see that I have removed the third brush, because when I install my electronic voltage regulator the third brush is no longer used. But you can see the adjustable slot and how it's positioned in relation to the generator mounting bracket.
It's hard to see in your picture, but it looks like you have a mounting bracket and a tension bracket. If so, your powerhouse probably was made with a 5 brush holder, but sometime in it's life the brush holder was broken and replaced with a 3 brush holder. That's OK, and it will work just fine.
When you are looking into the rear of the powerhouse with the mounting bracket down at the 6 o'clock position, then the slot for the adjustable brush should be in the 9 o'clock position.
I should also add that a couple of times I have found the ground brush post didn't make a very good connection with the metal mounting strap. So, now when I work on anyone's powerhouse I use an Exacto knife to scrape the base of the post and metal next to it very clean, then I use a 250 watt Wen soldering gun to solder the post to the metal. Try to work fast so you don't overheat the fragile bakelite ring. You just need a small amount of solder, but it needs to be hot enough to make a good joint, and not a cold solder connection.
I see your powerhouse is an early one that has the cutout mounted on the side. In this case, then the 3 brush holder that's on it now needs to have the adjustment slot at the 6 o'clock position. So, just turn your brush holder 1/3 turn clockwise.