Logo: 'Security with Interest'
Britain's First Municipal Savings Bank relates what happened next:

A large number of designs were submitted, many showing marked initiative and ingenuity. The committee spent a considerable portion of one morning in examining and criticising the designs, and ultimately decided on the one submitted by Mr E G Barnes, which was duly registered.
His conception of a key with the simple but effective slogan "Security with Interest" appeared to meet the case. Little could he have thought that his design would be so prominently displayed and so well known as it is to-day. It explains in the simplest language what the Bank stands for. Other designs submitted had considerable merit, and were also accepted for propaganda purposes.

The logo was used extensively, including large versions that were fixed outside branch premises, and which were illuminated at night. In many of the pre-war photographs of branches, the key is hung at right-angles to the building's facade. Two exceptions are Aston Cross and Nechells, where the key is hung flat against the branch's facade at first-floor level; both of these branches also have the key design on window panels (a practice also adopted with the glass panels of the doors of the city's trams).

The externally affixed keys were removed from branches (possibly during the war?), but all branches came to have the design on their windows. It was also a feature of the internal decorations of the Bank's Head Office in Broad Street.
The design also featured extensively in copies of the Bank's Annual Report. Curiously, the first two editions of the Report (1920 and 1921, where the key was on the back page) show the key with a rather strange-looking device protruding from the left side of the design. This was probably, therefore, the original winning design, as the Bank Committee had specified in the competition rules that the design should be suitable for affixing to lamp-posts or electric standards. An early example is shown at Image 059 - when the logo was used at the British Industries Fair at Castle Bromwich in 1925

The example shown here is a reproduction from the 1921 Annual Report.
An example of the modified logo (without the device on the left side), which was used in Annual Reports after 1921, is illustrated below and on this site's    Home Page
The key logo was also included in the design of branches by incorporating an engraved version on window panes, as can be seen at Cotteridge branch. An example of such a window is illustrated below (photograph by Christine Skett).
When the Bank's Committee of Management decided that the BMB would need a design that would become known throughout the city, the students at the School of Art (right) were asked to submit designs. The Bank's Chairman  (Councillor C T Appleby) offered a prize of Five Guineas (5.25) for the best submission, and the School of Art's director laid down three conditions for the design:

entries to the competition to be limited to designs  submitted by students attending the School of Art, or the branch schools;

the designs to be suitable for reproduction, if   desired, in enamel;
the Bank to own sole rights to the design.
Located in Margaret Street, Birmingham, the School of Art's
building was opened in 1885 - it was the first municipal school of art.