Some arms require that you flip them and hold them all the way up, so that they release the locking tang. As you're holding, a little careful wobbling back and forth while prying gently with the screwdriver/tool/thingy will usually pop the arm off the splined spindle pretty quickly.
The usual care is to be taken with the paint. Usually putting something like thin cardboard under the screwdriver so you don't scratch or chip the paint.
Assuming it isn't already pitted to the moon in that area from rock and bug hits over the last 50 years or so.
From the looks of it, I can't tell if the tool is designed around pulling the locking tang out, to unlock the arm, or if it's designed to hook under the arm head to pry it off the spindle.
Might do both!
After this many years, the arms might have been changed out multiple times already. Only you'll be able to tell if there is a tang or not. Or whether the tang backs out when you lift the arm. Or not...
Paul