The 1976 series, depicting architect Francesco Borronmini, psychiatrist Auguste Forel and others, was recalled in 2000.
The notes are no longer legal tender, but can still be exchanged for current banknotes – that is, until May 1st 2020 when any unexchanged notes will become worthless.
However according to the SNB's annual report there are still some 1.1 billion francs in 1976 banknotes out there, reported the Luzerner Zeitung on Monday.
Whether stashed in grandmothers' attics, saved for a rainy day or perhaps taken abroad by a Swiss expat hoping one day to return, the notes will shortly become nothing more than paper.
The 20-year time limit for the exchange of recalled Swiss bankbotes is fairly a unusual practice in Europe and one which should be quashed, according to one MP.
Manuel Tornare feels the limit penalises savers and has lodged a motion for it to be cancelled, wrote the paper.
But the Swiss government disagrees, saying in a statement last year that extending the 20-year limit would make old notes vulnerable to forgers equipped with the latest technology.
When the time limit is reached, the value of the outstanding notes will not be lost – it will be given to a national fund dealing with natural disasters.
The SNB is currently in the throes of releasing its ninth banknote series, unveiled in stages.
Unlike previous series, the new notes do not depict any particular person, but illustrate “a characteristic of Switzerland” represented by a place, an action or another graphic element.
The first in the series, the 50 franc note, took the theme of ‘wind' and was finally released in April 2016 six years later than planned due to problems with the paper supplier.
The next in the series, the 20 franc note, with the theme of 'light', will be unveiled on May 10th this year.
Older notes from the eighth series, released in 1995, remain legal tender until further notice.
The SNB's first banknote series was produced in 1907.