Do you have the exploded diagram diagram for the QC gear box to identify the part number of what you are missing. The lack of the detent pin will not stop the selector working.
I you cannot locate the diagram, or find one on an internet search, pm me.
My selector was sloppy, and was fiddly to use, turned out to be a loose screw.
Remove the cover from the gearbox and check the sliding gear works.
Peter