I have a well-used Rotring 600 multi-pen, the one with a mechanical pencil and red/blue ballpoint cartridges. It's about 15 years old. The part of the pen that you twist to retract the mechanism has broken from many years of use. I know these are no longer being made. Are there any shops that stock replacement parts? I love the pen and would really like to have it repaired. Any info would be appreciated.