I've seen adapters available on eBay for conversion of Parker 61 pens. It's basically the metal part with two screw threads on it that needs replacing with a black plastic version. I don't remember how much they were but it wasn't a hefty price tag.

My problem with Parker 61 pens is that the arrow can tend to come off in an ultrasonic bath and they are impossible to replace as perfectly as they were originally placed on, apparently with no adhesive.