The royal cipher on the front of the box shows when it was made. The functioning box has ER II on it, which means that it was installed after 1952, when Queen Elizabeth II became monarch. The redundant box has GR on it; the writing style suggests that it was installed in the 1940s, when her father, George VI, was King.
In the 1860s, the sub postmaster was also the schoolmaster. From the 1880s to 1920s, there were two deliveries a day on weekdays, and one on Sunday. Post was taken to and brought from the mail train at Bucknell Station by horse & trap.