See Scan for condition.   

Without lifting it is impossible to be 100% sure of wmk.

It is blued highly glazed and date also indicates SG 63.

Later examples not possible so can only be 62 or 63

Hard to find as an individual stamp, let alone on cover.

Markings on reverse are superb including Calais "angl' transfer stamp, and backstamp of Lyon.

Interesting reversed Pd stamp caused by being placed on a newly stamped wet Pd stamp on the envelope beneath it.

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