The two lamps on either LHS and RHS of the buffer beam is for an express passenger train (Source).BUFFER BEEN OIL LAMPS.
Some version of original W1 in gey have go the two lamps for expels passenger ,but some have got three along front buffer beam and one above smoke box door. What is that signalling can not see it on lamp signal charts .
From memory; the 'all filled in' code (i.e. smokebox + 3 on buffer beam) was reserved for the royal train, however I am struggling to find a source for that one for some reason....