I think you're probably looking at a slot that runs from the mast base to the nose. That was just there because of a German government rule that requires boards to have a secondary attachment to the rig, to stop them blowing away if the uni failed. They used to have a bit of shock cord there when delivered. Just ignore it.

Are you talking about the mast step sliding in a track? As Chris said, the groove in the foredeck was just for a safety leash but some photos I've just looked at show the 600's having an adjustable mast track. These tracks have a foot-button to adjust them but the internal workings can be different so if something's broken inside the track it might need some closer photos to solve.

Extract from the 1985 HiFly brochure
The rope is a a safety leash but is not strictly speaking needed. It is not mentioned in the brochure but if you want it get some shock cord and run from bow attachment point to universal base above the rubber joint. Occasionally the uni would pop out of the track or fixing point or break so this preserves the rig and stops it separating from the board. Can be a good thing if it happens offshore or in a big waterway. Beware of the shock cord, because it can hurt you when the rig pops out !