Brouwer, J.W.L. “P. Van Der Steen, Cals, Koopman in Verwachtingen 1914-1971”. BMGN - Low Countries Historical Review 121, no. 2 (January 1, 2006): 366–368. Accessed November 24, 2024. https://bmgn-lchr.nl/article/view/URN%3ANBN%3ANL%3AUI%3A10-1-107939.