The possible cut mark (or bench mark) seen close up here SU1484 : A mysterious mark, London Street, Swindon is visible just this side of the doorway in the first course of the stonework beneath the cill. The building is part of what was once the Great Western Railway's main factory established by I K Brunel in the mid 19th century. I think this was the carriage and wagon works.
Sadly, this attractive clock has been removed and put into storage pending changes to the area where it was situated. When and where it will re-appear is anyone's guess.