Auxiliary handstamp, FIRST CLASS MAIL ENCLOSED
Personal communications, such as cards or letters, were required to be mailed "First Class" at that postage rate and were not permitted to be enclosed within Second or Third Class matter. They were allowed to be inserted into Parcel Post or Fourth Class packages if the sender declared their presence when mailing the item and paid a supplemental charge. When the sender paid the supplemental charge, usually the one-ounce letter rate in addition to the parcel rate, the postal clerk applied the "FIRST CLASS MAIL ENCLOSED" auxiliary marking to the package. The mailer could also write this endorsement on the package.