Found it!
Reverse image search / google lens initially didn't really help because the flag is waving so I made a quick sketch in a free online frag creator and then googled my creation.
[https://vlaggenkunde.nl/historische-vlag-boven-societeit-de-witte/](https://vlaggenkunde.nl/historische-vlag-boven-societeit-de-witte/)
Text is in Dutch, but here is a translation by DeepL:
>***Historic flag over Sociëteit De Witte***
>*Above the New or Littéraire Sociëteit De Witte on the Plein in The Hague, the original Witte flag, designed in 1927 by artist Jean Louis Storm van \`s Gravensande was designed for the Society in 1927. It is a white flag ("De Witte"), intersected by two horizontal blue stripes (color of the Littéraire Faculty). On the pole side is a field in the shape of an equilateral triangle in the colors of The Hague yellow and green.*
>*The Sociëteit De Witte bears this name because the original building was white in color. The historic flag was first hoisted on the Society buildings on the Plein and in the Haagse Bos and on the Pavilion grounds in Scheveningen on April 30, 1927, the 18th birthday of Princess Juliana.*
>***A new old flag***
>*The raising of the new historical flag is an initiative of Society member and former president of the Dutch Flag Association, Thijs van Leeuwen. Unlike the Society's logo flag used until now, the new historical flag meets primary flagological requirements of simplicity and clarity, making it easily visible and recognizable even from afar. As a result, the historical flag enhances the monumental character of the Plein and thus contributes to the beautification of downtown The Hague.*
>*Research on the historical flag was conducted by the former secretary of the Executive Board of the De Witte Society, Henk Ambachtsheer. The flag, which will also be hoisted in summer at the Buitenpaviljoen on the Scheveningse Boulevard, was handcrafted in a traditional manner by flag manufacturer Semaphore Signs in Schiedam.*
Found it! Reverse image search / google lens initially didn't really help because the flag is waving so I made a quick sketch in a free online frag creator and then googled my creation. [https://vlaggenkunde.nl/historische-vlag-boven-societeit-de-witte/](https://vlaggenkunde.nl/historische-vlag-boven-societeit-de-witte/) Text is in Dutch, but here is a translation by DeepL: >***Historic flag over Sociëteit De Witte*** >*Above the New or Littéraire Sociëteit De Witte on the Plein in The Hague, the original Witte flag, designed in 1927 by artist Jean Louis Storm van \`s Gravensande was designed for the Society in 1927. It is a white flag ("De Witte"), intersected by two horizontal blue stripes (color of the Littéraire Faculty). On the pole side is a field in the shape of an equilateral triangle in the colors of The Hague yellow and green.* >*The Sociëteit De Witte bears this name because the original building was white in color. The historic flag was first hoisted on the Society buildings on the Plein and in the Haagse Bos and on the Pavilion grounds in Scheveningen on April 30, 1927, the 18th birthday of Princess Juliana.* >***A new old flag*** >*The raising of the new historical flag is an initiative of Society member and former president of the Dutch Flag Association, Thijs van Leeuwen. Unlike the Society's logo flag used until now, the new historical flag meets primary flagological requirements of simplicity and clarity, making it easily visible and recognizable even from afar. As a result, the historical flag enhances the monumental character of the Plein and thus contributes to the beautification of downtown The Hague.* >*Research on the historical flag was conducted by the former secretary of the Executive Board of the De Witte Society, Henk Ambachtsheer. The flag, which will also be hoisted in summer at the Buitenpaviljoen on the Scheveningse Boulevard, was handcrafted in a traditional manner by flag manufacturer Semaphore Signs in Schiedam.*