As I understand it there are the following 51 aero connectors (for full size Mark I pens): 1) Threaded shroud, no O-ring groove. 2) Slip fit shroud, no O-ring groove, no shoulder for barrel recess. 3) Slip fit shroud, O-ring groove, no shoulder for barrel recess. 4) Slip fit shroud, O-ring groove, shoulder for barrel recess. Did I miss any? Is it correct that I can: - Use an o-ring hood on a non-o-ring connector (without an o-ring of course)? - Use a recessed barrel on a non-recessed connector? If so, can I use Connector #2 to replace #2, #3, and #4 ? How about using #2 to replace #1? Can I press fit a threaded shroud onto connector #2? Maybe sand down the diameter of the connector a little first? Thanks, Brian