There was a late fee system in London from the 1790’s. By the payment of an extra 6d a letter could be handed in 7:00pm and 7:45pm at the Chief Office of the General Post after the closing of the mails and secure its despatch the same night.
From the late 1850’s to the 1870’s letters can be found with ordinary cancellations , but with an extra postage stamp in the top or bottom left hand corner of the envelope. This would appear to have been the way of paying the extra late fee charge.
There was a marked increase in the number of late fee letters handled by the Post Office when in March 1880 the ordinary late fee charge of ½d was introduced for the whole of the British isles. Late Fee marks with a hexagonal dated portion had begun to be used at a few large London offices in the late 1870’s and occurred into the 1890’s
It is well worth a search of your collection as London Hexagonal Late Fee cancellations are very collectible items. I list them on my Ebay shop as and when I find them and they tend to sell very quickly. There make a great addition to a collection as they enhance late Victorian stamps which sold in millions.
The example above has a catalogue value of £35.00 but in fine condition is readily available to purchase for around £2.80. The above example off cover would command a premium of around 40%.