Japanese ps 'label'

[johnhbarker@btinternet.com]

Can any member provide information on this or similar items. It appears as a label measuring 124 x 74mm made up of a sheet of pale blue paper, folded down the middle, punched at the top and reinforced with thin card ring through which there is a thin wire fastener. There is a gummed flap at one side which seals the long side but leaves the end open. The indicium '20' is the same rate as for contemporary postal cards so I presume would not pay the postage for a package. Any information would be most welcome.