The hero Orrie Main begins his education at West Point in 1842. His first letter to Madeleine Fabray, presumably sent that Fall (before a subsequent snowstorm at the Point) bears a blue postage stamp. United States postal stamps were not introduced until 1847, following an Act of Congress.
When Cadet Orry Main knocks on Madeline Fabray's door in Charleston during his first leave from West Point, the house appears to have push-button (presumably) electric doorbells at both front and side doors. Wired house current had not yet been invented in 1844.