Here is a photo of one of the construction details not visible from the outside of the instrument.
Here is a view of the fingerboard extension.  Note the use of  a dovetail joint. This insures that the extender is joined to the neck,  rather than joined to the body.  This is similar to the connection used by
D'Aquisto on his archtop guitars.  I've adapted this to my mandolins to maintain the appearance of the
traditional 15th fret crosspiece.  Besides being very strong, this connection allows the extension to have minimal contact with the top plate.  Minimal contact with the top prevents the chance of the 15th fret fold up (the dreaded hump) that many ageing mandos suffer from.  This feature is unique to my mandolins. Others glue the extender to the top plate with a butt joint to the neck.