The numbers are just code numbers, the 600 series of codes for example identifying the Operation Torch vessels

SS620, 621 and 622 were part of convoy KX.2 and so were three of the following: Duke of Athens, Flowergate, Fort Senneville and Houston City

The SS and LS prefixes were used, I believe these stand for ‘Special service’ and ‘Liner Service’, the basis on which the vessels were chartered