I almost missed it again, it was so faint. And impossible to see on the unenlarged photo above. But there it is, inscribed into the cement rendering, while setting, at the foot of a canalside wall:
MAY 1ST 1956.
C.N.R. (?) HOLMES.
Wow! That graffiti - if I can call it that - was done before I was born! Presumably messrs. Holmes and Webb were two of the builders responsible for the block of flats above.