Spanner screwdriver: so that's what it's called! I recently had to MAKE one by grinding away the center of the tip of a cheap screwdriver. I'm sure I've adjusted the verticals on an Underwood portable without special tools, but I think I had to remove the shell.
The confusing thing is on wikipedia, where all the screw drive types are listed, the spanner screwd looks like a regular screw two holes instead of a slow in it—like a European electric outlet.I've found a set online that I think should cover the range of Erikas, perhaps with a little grinding away the middle slots for the largest one.You can use any wrench the right sizes, provided it's flat enough. We couldn't find a newly made wrench that was flat enough to fit.