I have found some plotter pens. Ons pack says Calcomp 104x, 1023. The other I can't trace. They look like they clip into ring. Pics here.
https://1drv.ms/f/s!Ag4BJfE5B3onnqhG_uReQsCndYafvg?e=WpT5nw I have a few more of the Calcomp, packets of the un-branded sort. Free for the cost of postage Dave
