Hello there @Amiroquai
I can’t tell you the exact number, you need, but I’ve had luck with a small multi-tool kit which has several sizes. It just seems to be a matter of finding the one that doesn’t wiggle around too much and fits tightest in the screw hole. Like many things in life, a tight fit seems to work best (lol).
My Hohner Special 20 seems to work best with this bit which says 3.5 on the barrel, but it’s a flat head screw, so that’s not much help:
I had a similar but different problem when I stripped the holes on my Easttop harp. Since I was unable to find any sleeve screws, I ended up getting some twine and wrapped it tightly around each end. This actually works out pretty well to keep the screws tight. I also think it looks pretty cool, but that’s up for dispute. Here’s a picture of the Blue Blazer along with some of her friends.
I think that just as long as the plates are tight together, that probably just about anything could work. May not look too pretty, but I think most of us are interested more about how the harp sounds than how it looks.
This is the “tool kit” I use along with a small piece of wood to tweak the reeds. It all fits inside the pen-like barrel with a screw top and the bits fit into the other end with a magnet.
My numbering system may be different. As I mentioned, I use the V 3.5 flathead for my Hohner Special 20, and it looks like the V 2.0 will work for my new Hohner Golden Melody, which I have yet to open up. I was surprised to see that it isn’t the same size as for her sister. I just took it for a test run for when I do need to open it up. For my Lee Oskar, the PH 1 seems to fit the tightest. Not sure if this is equivalent to the pozi #1 that you mention.
Since you will likely love the harp and be playing it like a BOSS with the direction of our fearless leader @Luke, if I were you, I would probably get a set which has several sizes, that way you are ready for whatever harp you choose to add to your collection next. 
Just make sure you get a tight fit and you should be good to go 
