yes well thats the problem getting it apart?? It seems to be pressed together.
correct, some are pressed some are glued and some are even melted togther. all depends on the Brand.
Jap ones from the 70s are assembled with a solvent that melts the plastic and leaves no residue but they can often be split with a very carefully executed hacksaw blade on the seam as the melt isnt usually complete.
Mudnut is on the money as first the trip knob needs to be removed and again it depends on what breed as to how it comes off. Euro are usually metal cased and after the trip meter is removed its a fairly easy process to remove the cover.
metal case rings are crimped the metal is very soft so go gently.
after a good clean and a drop of lube you can reassemble with a good glue and a a wipe of black silicone to hide your scar and make it almost invisible.