The vessel in the picture from the 1950s is the earlier design of ferry, from the 1920s.
In the Kowloon Terminal there is a collection of photographs that illustrate the history of the Star Ferries, and this is one of the photos which shows a ferry like the one you describe. Anyone in Kowloon will be able to find this and give more details than I can remember. There was also a bookstore nearby where you could buy postcards of this photograph.
Hope this helps, Gemma
Date picture taken