The City Bank were Bankers trading, according to a Post Office Commercial Directory of 1865, from:
with a branch office in:
I believe this bank became part of the Midland Bank group, which in turn became part of the HSBC group.
1868. Unofficial underprint type 9 (over the gum) in black.
PP20 4d vermilion (SG 94 or J57)
There are a number of later perfins that have also been identified as belonging to this firm. I do not have illustrations of these perfins but they are referenced in Tomkins as:
See the Philatelic Links and References page for more information on Perfins.