At the request of production designer Eve Stewart, writer and director Douglas McGrath advanced the time from the 1830s to the 1850s, so she could incorporate elements of the Industrial Revolution in her design plans.
Though the novel was originally set in the 1830s, this version was moved to the 1850s for set design purposes. The postage stamp was introduced in 1840, so it is perfectly correct to appear on the letter from Nicholas (Charlie Hunnam) to his Uncle Ralph (Christopher Plummer).