The story
The Commonwealth and Dominion Line was formed as an amalgamation of four shipping companies on 23 January 1914, J. P. Corry & Company, Wm. Milburn & Company, Thos. B. Royden & Company and Tyser & Company. All four companies had operated routes to Australia and New Zealand, and together formed a new shipping line of 23 ships. Milburn & Co brought nine ships, Tyser & Co. eight, Corry & Co. five and Royden & Co. three. The company's head office was at Tyser & Company's former officers at 9-11 Fenchurch Street in London. The board of directors was drawn from the directors of the constituent companies and was under the overall chairmanship of Walter Tyser. Representing Thos. B. Royden & Co. was Thomas Royden, later chairman of Cunard.
Reflecting the make up of its constituting companies, the ships of the new line adopted a varied arrangement of existing styles. The ships flew Tyser's house flag but had Corry's funnel colours of buff with black tops. The overall scheme followed Tyser's patterns of grey with white superstructure and masts, while Milburn & Co's practice of naming ships with the prefix 'Port' was extended to new ships then under construction for the company, and then expanded further in 1916 by renaming all existing ships under this style. This practice led to the name 'Port Line' coming into unofficial use.
From Wikipedia’s article on Port Line, shared under CC BY-SA 4.0.3