Sorry to dig up this old post, but this needs slight correction
Class 253 sets during the 1980s were formed :
TF+TF+TRUK+TS+TS+TRSB+TS
Later these were revised to :
TF+TF+TRSB/TRUB+TS+TS+TS+TS
Hence, why there are two First Class coaches visible
It wasn't until the mid 1980s when the TGS vehicles were added that one of the First Class coaches was dropped on all the Class 253