Postal Stationery Society

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.

I have had a couple of replies to this query which appear in the February 2025 Issue of the Journal (#113).
These items are 'Parcel Postal Cards' issued between 1951 and 2003 and used to send a message to accompany a parcel, during this time it was not permitted to enclose written correspondence within a parcel.