This is a British commercial overprint used by the London-based department store business Marshall and Snelgrove which subsequently merged with Debenhams.
A "commercial overprint" acted like a perfin to deter theft of the stamps by employees as for many years mint stamps could be cashed in at a postage office.
Once the overprint was applied it could only be used as a revenue stamp and could not be exchanged for cash at a post office.
This stamp will have paid the stamp duty on a bill or receipt.
