This banknote is 50 British pound note , one of those millions of British banknotes that the Nazis forged in the barrack 19 of KL Sachsenhusen, following the plan of Reinhard Heydrich and approved by Heinrich Himmler. This plan, known as Operation Bernhard, had the goal of flooding British economy with fake money and lead it to collapse. All the forgers that worked in this operation were 140 specialized Jews in forging that were thoroughly chosen among st all the Jews in different concentration camps.For further information about this operation, that was also turned into an Acadent Award winning Hollywood film in 2008, please check the following link:The differences between the original and counterfeit note can only be seen on the watermark. In the bottom inscription BANK OF ENGLAND the letter N in the word ENGLAND is slightly different.The line extending from the letter N down in the original banknote is on the left, and in the false one exactly in the middle. Operation Bernhard - the code name of a secret German program developed during World War II to destabilize the British economy by admitting to the British market huge amounts of counterfeit notes of denominations of 5, 10, 20 and 50 pounds (exactly as issued by the Bank of England).Operation Bernhard is considered the greatest financial counterfeiting in history.The operation plan was developed by SS-Sturmbannführer (major) Bernhard Krüger, whose name became her code name. A team of 142 counterfeiters recruited from among prisoners of the Sachsenhausen concentration camp, Auschwitz and others worked under Krüger's command. The assumptions of the operation were implemented from 1942. Preparation of universal matrices for the production of banknotes, development of the appropriate type of paper (consisting mainly of fragments of cotton fabrics), copying watermarks and breaking the algorithm enabling the generation of correct serial numbers of banknotes caused many difficulties for the participants of the operation. However, until the evacuation of Sachsenhausen in April 1945, the printing press produced 8,965,080 banknotes with a total value of £ 134,610,810. The securities that were printed as part of Operation Bernhard are considered to be the most perfect counterfeits in history. It was almost impossible to distinguish them from the original banknotes issued by the Bank of England.The Bank of England was first informed by the spy about the plot in 1939. Bank officials first discovered the counterfeit currency only in 1943 and identified it as the greatest threat in history. The presence of counterfeits was first recorded in an English bank in Tangier (currently in Morocco). Until the 1940s, the issue of each banknote by the Bank of England was recorded in registers bound in leather covers, which were stored in bank archives. Officials in Tangier noticed that the banknotes in the series to be printed were already in circulation. 

A few notes for you regarding Operation Bernhard...hope find this useful...

Location - Barracks XVIII / XIX, KL-Sachsenhausen (1942)

Project Leader - SS Major Walter Bernhard Krüger

Professional Counterfeiter - Solomon Smolianoff

Currency Created - over 600 million dollars worth of British Pound Notes

Confirmed denominations - 5, 10, 20 and 50 pound notes

Unconfirmed denominations - 100, 500 and 1,000 pound notes

Ledger provided by Operation Bernhard team post war (used to confirm the serial numbers used):

Denomination 5 pounds (A128-A275, A281-A314, A317-A398) (B105-B131, B134-B182, B186-B237,B256-B279) (J373-J377)

Denomination 10 pounds (K102-K184, K187-K199) (L100-L107) (V105-V153, V163-V170)

Denomination 20 pounds (M43-M55)

Denomination 50 pounds (N42-N61)

The Bernhard group also worked on counterfeiting the US 100 dollar bill (1944) and successful notes were produced although they have never been found in private collections.

Propaganda detailed postage stamps were also produced, with details including a small Star of David placed inside the English King's crown (3d value stamp)