The number on these is usually very small, with three digits to the left, and three to the right of the keyway. Seeing that it has been apart it’s quite likely that the digits have been lost when the rivets etc have been filled back. In which case only a general date is going to be possible.
Seeing that it has the Barrell & curtain it has to be post 1852, and with the St Pauls address it must be prior to 1877.
