Miss Prins, during the 1920s and 30s lived at 21, Clanricarde Gardens, London and worked as a secretary for Mr. Moses B. Cotsworth. He started the International Fixed Calendar League to reform the calendar into 13 months of 28 days. She translated leaflets and attended conferences on behalf of the League. Some of her letters survive in the League's archives in the Institute of Social History in Amsterdam. See also for her relatives:
http://www.willempoelstra.nl/For-Hanna