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@gbj_tester
The notches are to remove the freewheel bearing from the freehub unit, i.e. to separate the inner (wheel attached) part of the freehub body from the outer (sprocket attached) part. Don't touch them, you'll break it and never get it back together.
Not sure what Spengle did for freehubs, but that looks like a hybrid HG/Uniglide body minus the usual ¼" loose ball cup and cone bearing, so they may have butchered Shimano FH-6200s to make their wheels. If they did, you have another problem coming once you get the bearing out.
First step is to try to get the end cap off the NDS to see what's up there, presumably another cartridge bearing pressed into the hub shell with nothing else retaining it.If that's so, you can drift the axle out from either end, which will leave the other bearing in situ, then drift out the bearing you have left in place.
What you're probably going to find then is a freehub body attached to the hub shell with a large hollow bolt, similar to the one with which you'll be familiar if you've ever taken a common Shimano HG freehub apart, but instead of the 10mm double hexagon inside which you take off with a 10mm Allen key, there is a spline drive which needs the specific Shimano TL-FH10 to undo.
The bolt is captive in the freehub body (not loose like common HG freehubs), and it has a weird twin-start thread which takes the drive from the freehub rotor to the hubshell, not the fine pitch single start thread which, on HG hubs, serves only to clamp the freehub rotor drive spline to the mating drive spline formed in the hub shell.
TL;DR
You're not going to be able to convert this to fixed, which I suspect is your real question.