"Haven of Rest" by Douglas Gurton says
"LLANSTEPHAN CASTLE was later joined by two fairly modern cargo ships of the same company, the BRATTON CASTLE and SANDOWN CASTLE. These were known as "black ships" being painted black in contrast to the lavender grey hulls of the passenger liners.
Dates are from [George Swieszkowski]. However:
Northern Whig of 26 Mar 1923 reported BRATTON CASTLE arrived Gravesend 25-03-23 from Bombay
Aberdeen Press & Journal 22 Aug 1923 reported BRATTON CASTLE sailed Middlesbrough 20-08-23 for Cape Town
Sold to Kulukundis Greece 1931 renamed PROTEUS.