One side of the coin is dated "28 IX 74" which corresponds to the museum's opening.
It is a shiny silver color and has been represented to be sterling silver, however no silver hallmarks are visible on this tiny coin.
It is about 17 mm in diameter (just a bit smaller than a US dime), and has about a 2.2 gram mass.
Nothing more is known about this coin, and it seems quite rare. If it is truly composed of sterling silver, then it is likely worth about $15.