2001 Canada Maple Leaf Medallion - Various States Of Coating

This RCM 'map' set from 2001 has always intrigued me, primarily for the medallion and the coating/film on it (as it is the only one the RCM ever produced) but also that it is much less commonly available than its millennium predecessors. To me, this medallion and set should be treated just like the 1999 and 2000 Nestle medallions from same-names sets, which are always less commonly available than the normal millennium sets.



Finding a set with a medallion with pristine coating is always a bonus, like this



I have gone through countless over the years and obtaining one that is still with a thick and complete coating, let alone coming in mint-condition packaging, is very far between.

Typically, the coating has started to come off in various degrees (even in the original packaging) or it has become thin, showing the underlying copper-nickel through, like this




And sometimes for whatever reason, people will clean/wipe/polish the coating off completely, like this,



and reverse




Does anyone know what the coating/film actually is?
Cheers,
BK