Very good chance that different make masts and booms were fitted over the years also possible yours is not original.
Post a picture of yours then there's agood chance someone will recognise it.
What is your problem exactly?
Look around at other Mirrors. Not uncommon. Two or theree in the yard I'm in
Generally goose neck allows boom to swing and to lift and fall. Fitting is either fixed in position in the mast or slides in a track to allow tensioning of the luff.
Mine has a gooseneck which slides in track on the mast in which the sail bolt rope runs.There is a clamp screw to hold it in position.The goose neck has a universal joint with a spigot which inserts into the boom.
The boom can then swing vertically and horizontally.I added a vang.Look here. lots info on mirror and other small sailboat, http://groups.yahoo.com/group/smallsailboatclub/