How can I remove the small slitted nuts to open an old rear hub?
With an angle grinder and a flat file you can easily make a tool for this purpose from flat iron (preferably hardened):



If you use hardened flat iron, make sure to spray or dip it in water regularly too not lose the tempering when grinding. The small tip that fits into the slit of the nut needs to be finished with the flat file since the angle grinder doesn't reach to make it the right angle.