While Victorine Hefting tentatively confirmed the attribution to Jongkind in 1983, Saskia de Bodt has, on the basis of photographs, kindly confirmed the attribution.
(1) The Hoofdpoort at Rotterdam was located at the entrance of Rotterdam harbour and torn down in 1856. Jongkind grew up in nearby Vlaardingen, and the Hoofdpoort would remain one of the artist's favourite subjects from 1856 to the the mid 1870s ((V. Hefting, Jongkind, sa vie, son oeuvre, son poque, Paris, 1975, nos. 146, 171, 199, 233, 289, 538 and 638), though all taken from another view point than the present drawing.
(2) Leiden is close to The Hague, where the artist worked until 1845