From Stamps of the World
Holyhead is the largest town in the county of Isle of Anglesey in Wales. It is also a community and a major Irish Sea port, serving Ireland. Despite being the largest town in the county, it is neither the county town nor actually on the island of Anglesey. Holyhead is located on Holy Island. It was originally connected to Anglesey via Four Mile Bridge, so called because the bridge was four miles (6 km) from Holyhead on the old turnpike Road. In the mid 19th century, Lord Stanley, a local philanthropist, funded the building of a larger causeway, known locally as "The Cobb", it now carries the A5 and the railway line. The A55 dual carriageway runs parallel to the Cobb on a modern causeway. The post road built by Thomas Telford from London strengthened Holyhead's position as the port from which the Royal Mail was dispatched to and from Dublin on the Mail coach. The A5 terminates at Admiralty Arch (1822–24), which was designed by Thomas Harrison to commemorate a visit by King George IV in 1821 en route to Ireland and marks the zenith of Irish Mail coach operations. With the opening of the railway from London to Liverpool, Holyhead lost the London to Dublin Mail contract in 1839 to the Port of Liverpool. Only after the completion of the Chester and Holyhead Railway in 1850 and the building of Holyhead railway station did the Irish Mail return to Holyhead, operated from London Euston by the London and North Western Railway.
Holyhead was allocated the Post Office 374 Numeral.
LONDON - HOLYHEAD TPO
The London to Holyhead Travelling Post Office was set up to manage London mail that was destined for both Ireland and the America's. Mail was sorted on the journey for onward destinations.