{Difficult to read but, for example:}
365 PLYMO
227 ARMM
430 7961 G
Site: Police bricks (1 memorial)
EC1, Myddelton Passage
Brought to our attention by  which has a very good, thorough post about these strange bricks.  The graffiti is apparently mainly the collar numbers of policemen but some have added dates and initials.  also reports on them giving dates around 1890-1910, but says "The origins of the custom remain obscure; boredom may be sufficient explanation."
The 'G' in many of the texts apparently indicates that the officer was from Finsbury Division, based at the King's Cross police station.
The wall is north-facing which makes it difficult to read, or photograph, on a sunny day. Probably best visited early or late, or possibly on an overcast day.
Two other instance of brick carved memorials: Emma Bowden and WW2 GIs.
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