[Merry Go Round]

Hi, I am reading "The Modellers & Enthusiasts Guide to The Somerset & Dorset Line" by Brian Macdermott and at the back of the book he includes extracts from the 1961 WTT but I would like to know what the annotations next to the service are. There are 3 which are SO SU and SX, any help would be appreciated.

 

[Merry Go Round]

Thanks for the replies and I will pop over to Mremag. S must have been for Saturday as regular trains didn't run over the S&D on Sundays

Thank You