Pre-adhesive mail
Pre-adhesive mail, also called pre-stamp mail, are letters carried in mail systems before the issuance of postage stamps. A stampless cover is another description and generally also refers to any item of mail sent before the issuance of postage stamps but it can also refer to mail sent, after the introduction of postage stamps, unpaid (as was permitted in many countries) or without the pre-payment being indicated by the affixing of a postage stamp; it could have been pre-paid in cash and marked paid.