I still have one of these and I am giving up on them. Have replaced two of them and will replace the third and last this year. Gone/going to Ongaro arms which have, to me , a superior spline system with a clamp screw.
I grease all, the spline, the screws as I assemble. Otherwise corrosion will freeze the arm to the spindle spline.
I found the Anco hard to locate in the last few years.
i think what you have is the common 1/2 nominal. Your motor is likely the American Bosch, model WWF, which is what I too have.