This plate could not just be a souvenir because there was only one minted. Yes it could be a commemorative seal but it could not be attached to a band because there is nothing where to attach to it.
If you notice on one side of the plate, above the engraved building of the House of Diet (the seat of power of Japan) and that building is also the house of representatives of japan, the japanese encription means "AKIHITO HEISEI HOUSE OF PEERS". (like the brithish house of lords). Akihito was renamed Emperor Heisei. He is the present reigning Emperor now after the death of his father Emperor (Hirohito) Showa in 1989. This perhaps explain why the English encription.
On the otherside of the plate, if you notice below is an engraved name of the owner in japanese characters, It could not be read properly if it is Hirohito or Akihito's titled name.
The House of Peers originally comprised: (1) all imperial princes (shinno) and lesser princes of the imperial blood (o) over the age of twenty, (2) all princes and marquis over the age of thirty, (3) 150 elected representatives of the counts, viscounts, and barons, (4) 150 additional members nominated by the Emperor, in consultation with the Privy Council, and (4) 66 elected representatives of the 6,000 highest taxpayers. During the first session of the Imperial Diet (1889-890), there were 145 hereditary members and 106 imperial appointees and high taxpayers, for a total of 251 members.