Thanks Ted. I have the 9000, really is the 6000 that I'm interested in. From what I was able to find online the 6000 is either 2.1 or 2.3 mm diameter, so I think it is probably too small for what I am needing. Probably will just have to buy one and see.
My problem is that the hole for the very tip of the pen is sized for a standard parker refill, but the bushing for the tip is too narrow for the wider body of the standard parker refill. After a little looking I am now thinking that the Fisher PR4 might be an option. The refill options are pretty complicated once you stray from Cross or Parker, but hey, at least I'm learning a little!