If it really was on a wall in 1979.

A manky wall, somewhere in Islington
Note the following:

a) RW 4 RW. Initially cryptic, is easily understood when one considers the narcissism of Roger Waters.

b) Whilst sadly the crude NF written upon the wall is probably for the National Front, nasty racist group that they were, it is also possible that it is short for NO FLOYD!

c) Some wag has made a Chad like Syd Barrett joke, not comprehending how much worse the album could have been if Syd had been there. Well, at least bad in a different way.

d) Oh that I was a boy and could piss so magnificently.