The 3-line auxiliary mark is under the stamp and reads something like:
RETURNED FOR POSTAGE N. S. M. D. No 45 MAR 4, 1953
It was applied by the Nixie Section, Mailing Division's clerk 45 (or whatever clerk number) to the unpaid envelope which was returned. The sender applied a stamp over the marking and remailed it.
The "Nixie Clerk" deals with all kinds of mail which needs extra help. That could be marking postage due mail, checking for valid stamps, address correction/completion, forwarding, returning, reading bad handwriting, repairing damaged in handling mail, etc., etc.
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