Great Britain #517 1.5d Deep Green Queen Elizabeth II, 1952-1954 Wilding Issue, Tudor Crown Watermark, A Single Usage on a Fine Undelivered Local Cover
A fine cover to Nightingale Grove in London, sent from Lewisham on June 20, 1955 (before the St. Edward's Crown watermark stamps were issued). As indicated on the front of the envelope the postage was underpaid by 1/2d, but two attempts at delivery were made, with the postal clerk noting the attempts in the upper left corner of the envelope. Finally, it was stamped with the large "1d Postage Due Return to Sender" handstamp, before being returned to Lewisham on June 22, 1955, as indicated by the backstamp. There is one other address on the front, which I believe to be a return address and some notes on the right in pencil, which I believe were added after the fact.
Our estimate of the value of this cover based on the above factors is $5. To understand how we have assessed the cover and arrived at this estimate and to gain a broader understanding of postal history collecting, I would suggest you read the article linked here. It should clarify the factors that make covers, and this one in particular scarce, and will help you to understand why we have estimated it at this amount.