After Thomas de la Rue replaced Shah's bust with the Mosque, they applied a yellow "Lion and Sun" seal over Shah's WMK, printed, and send the notes to Iran.
Furious to see the "Lion and Sun", which was a representation of Royalty, the Central Bank placed a second order, and the next shipment arrived in Tehran without the yellow "Lion and Sun" overprint.
In both groups when the banknotes arrived in Iran, the Central Bank applied the black circular seal on the front and back of each note.
Therefore, Pick 131 exists with and without Yellow under print, which can be detected much easier by looking at the back of each note. See attached image.