British Nigeria

The badge of this protectorate has a red field, upon which are imposed two interlocked triangles in the form of a six-pointed star. In the center is the crown of the British Empire.

The union jack is the national flag of the colonies as well as of the mother countries and, although it is a rule observed more in the breach than in its observance, no other flag is to be displayed ashore. According to British flag law, the union jack, in its plain condition and without emblazonment or badge, is the only flag an individual or corporation in British realms may properly fly. However, since the shipping of the principal colonies is accustomed to fly the red ensign with the badge of the colony represented in the fly, this flag is frequently, if not indeed usually, displayed by the people of the several colonies as their particular flag. Vessels bearing colonial governors or other administrative officials of badge-possessing rank fly the union jack with a badge of the colony placed within a wreath at the intersection of the crosses. Vessels of the colonial public service display the blue ensign with the badge of the colony from which it hails in the fly.
from National Geographic, Oct.1917 [gmc17]
Josh Fruhlinger 13 February 1996

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I came across an explanation in one of the 1949 editions of a magazine called Nigeria, which had an extract from a letter written in April 1940 by Lord Lugard.

The design of the interlaced triangles is I think commonly called Solomon's Seal. I do not know if and when it was adopted as the seal of Islam but it was found on the lid of a very handsome goblet or jug of brass and copper covered with designs, which was captured by the troops when the Emir of Kontagora, the principle slave-raider in Northern Nigeria was defeated. I thought it an appropriate badge for Northern Nigeria and as far as I can remember it was my own suggestion. On amalgamation of North and South it was adopted as the emblem of united Nigeria.

D.Prothero, 23 March 1998