Great Britain 1840, 1d Penny Black, SG2, used, four margins, red MC

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Great Britain 1840, 1d Penny Black, SG2, used, four margins, red MC:
$116.73

A very nice used copy of the Penny Black. SG2 I suppose. Four margins and cancelled with a red Maltese Cross
Condition: see scan.
I do not know whether the mark on the back is an S (maybe a dealer\'s mark?) or whether it\'s supposed to be a 5, pointing towards it maybe being a plate 5 copy?

On 26-Oct-17 at 17:00:35 BST, seller added the following information:

PS: I have since been told that trh mark on the back is indeed a 5 and that this stamp is from plate 5.