The markings on the pole serve to indicate the ownership of the pole and allow engineering staff to determine how deeply it is buried. The GPO Mark is 10 ft from the butt end of the pole or 3 metres for PO or BT poles. The pole should be buried at a depth of 1/6 of it height.
Underneath the ownership mark, GPO (General Post Office), PO (Post Office) or BT (British Telecom), the height of the pole in the GPO era is in feet and after that in metres. The height is followed by a letter indicating the strength, L for Light, M for Medium and S for Stout.
Below the height and strength are two digits indicating the date the pole was first creosoted. The tree may have been felled and dried for some period before its first creosote protection.
A fourth line, usually found on BT era poles is a suppliers code.
Examples
As a child I can recall the pole feeding my parent's house having its arms removed and replaced by a ring-head mounting for the insulators. Much to my surprise Google Street View showed the same pole with the notches for the arms visible. The photograph below, taken on 28-October-2021, shows the 32 ft. pole was dated 1933, still going strong aged 88 years.
Below are poles from the three eras. The GPO became the monopoly supplier of the telephone in 1912. On 1st October 1969, the General Post Office ceased to be a government department and became the Post Office Corporation (PO) until 1981 when the postal and telecommunications split. Before this split, the Telecommunications Corporation became known as "British Telecom part of the PostOffice". On 6th August 1984, British Telecom was privatised, but part government owned until December 1991.
The three pole markings show, left to right. GPO 40 foot Medium 1954: PO 11 metre Light 1973: BT 11 metre Medium 1993.
