Arms consisting of a triple-towered castle with a golden key dependent from the centre were granted to "The Most Loyal City of Gibraltar" by Ferdinand and Isabella in the year 1502. The original grant on vellum is now in the archives of the Municipality of San Roque.
The words usually found underneath the Arms are "Montis Insignia Calpe", meaning "Badge of the Rock of Gibraltar" and these words also appear on the Colours of the Suffolk Regiment, which Regiment obtained permission to have the Arms of Gibraltar inscribed on their Colours in recognition of the gallant part they played in the Siege of Gibraltar. The words "Nulli Expugnabilis Hosti" meaning "Conquered By No Enemy" also occasionally appear under the Arms of Gibraltar, having been copied from the Official Seal of the Colony which consists of a picture of the Rock with these words underneath.
In 1869, a Circular despatch was sent to all Colonies requesting that a sketch of the Arms or Badge to be adorned in the centre of the Union Jack used by the Governor should be supplied to the Colonial Office. The sketch which was submitted by this Government on the 1 st January 1870, depicted the triple towered castle with dependent key with the words "Montis Insignia Calpe" underneath. This design was approved by the then Secretary of State and concurred in by the Lords Commissioners of the Admiralty.
In 1875 a copy of an Admiralty Flag Book was forwarded to all Colonies with an enquiry as to whether the devices shown in the centre of the flags were correct. It was also stated in the same despatch that in those Colours in which Badges for Flags had been approved which differed from the Armorial Bearings or device shown on the Seal of the Colony, no objection would be offered to their continued use but that it was hoped that the first opportunity would be taken of adopting the device on the Seal. The reply to this despatch was to the effect that the Badge for the Union Jack shown in the Flag Book was correct but that it differed from the device on the Seal of the Colony and that the earliest opportunity of adopting the latter device would be taken.
The origin of the device on the Seal of the Colony which consists of a picture of the Rock with a sailing ship in the foreground and the words "Nulli Expugnabilis Hosti" underneath cannot be traced but it has been suggested that it has been taken from a Commemorative Medal of the Siege, which is believed to have been designed under instructions from General Eliott. No record of such a model can, however, be traced.
No action has been taken with regard to replacing the badge originally approved for the centre of the Union Jack by the device shown on the Seal and in his despatch No. 171 of the 28 th September 1926, the late Sir Charles Monro recommended that the Arms originally granted by Ferdinand and Isabella should be regarded as the true and proper Arms of Gibraltar and that the device on the Seal should be changed to conform with this as soon as opportunity offered. He also recommended that the words "Montis Insignia Calpe" should be regarded as part of the device.
The Secretary of State sought the advice of the Garter King of Arms as to whether the Arms granted by Ferdinand and Isabella could be regarded as the recognised ones without the necessity for a Royal Warrant and, if so, whether they could be placed on record officially with the addition of the motto "Montis Insignia Calpe". He was advised that this could be done without the issue of a Royal Warrant and it was suggested that a properly attested and accurate copy of the Grant of Arms in 1502 should be recorded at the College of Arms.
"An escutcheon on which the upper two thirds shall be a white field and on the said field set a red castle, and below the said castle, on the other third of the escutcheon, which must be a red field in which there must be a white line between the castle and the said red field, there shall be a golden key which hangs by a chain from the said castle, as are here figured"