Neil,
I checked the files section for the drawing of the Sherline threading attachment adapter for the U3 and saw what I think is the file you referenced but I could not open the file for viewing.
Do you know if the file is empty or corrupted?
I am going to contact Sherline directly to get information that may shed some light on this. I am sure I can figure this out myself but did not want to reinvent the wheel if I can avoid that.
Thanks for your help.
Dick
OldToolmaker