Just after the Second World War, a major proposal was prepared to extend and automate the Melbourne Mail Branch. While the proposal to extend the building was not accepted, most of the automation aspects were adopted.

Probably the most significant effect was the introduction of a mechanical letter sorting machine which became known in Post Office parlance as the "Flat Top Sorting Machine".

Developed in the early 1950's, it was originally installed in the Melbourne Mail Branch. Machines of this type were to continue in use in the Spencer Street complex until its closure in mid 1989. Nearly forty years later, they were still in use in the Adelaide, Brisbane and Perth Mail Exchanges and will continue in operation until the completion of the OCR installation programme.

It is worth spending some time on the logic of this machine's operation for comparison with the later Letter Sorting Machines which were computer controlled. The "computer" was still the mail sorter as the age of computers was still a human generation away. Work study exercises had determined that when an operator was required to sort to more than ten positions (or "breaks"), the sorting efficiency fell by 1/60 (say 1.6%) for each "break" added.

The "flat-top" machine comprised a long horizontal table, hence its name, over a series of narrow belts running the length of the table. At each operator position, a grid of slots was placed. The operator placed the envelope into the appropriate slot where it was carried along on the belt to a stacker at the end of the machine. The letters in the stacker were regularly cleared. They were then either taken to another similar sorting machine for resorting (a secondary sort to enable a greater number of breaks) or bundled and tagged for transportation.

The number of sorting fields in the geographical area serviced by a major mail exchange usually required several hundred sorting breaks. The physical limitation of the average reach of a mail sorter would limit the number of breaks on each sorting machine to about 40. At this size, the operator would have sacrificed two-thirds of his speed. It was also noted that the missort rate grew with the increase in the number of breaks being handled by the operator. Typically, 24 breaks were used on each machine with seating up to 30 sorters.

At that time, the final sorting for suburban Melbourne required some 95 destinations or breaks. With a machine of 24 sorting breaks, four sorting machines were required to cover the full suburban requirements.

To sort to finality in Victorian Country would require in excess of 2,500 breaks which would involve a large number of machines with each final break on average carrying a very small amount of mail. The Country was also broken up into a like number of breaks to a Suburban with major centres having their own "slot" but the remainder of the State covered by Districts based on Post Offices that would sort to finality for the smaller destinations.

While the individual sort rate achievable with the "flat-top" was quite high (1500 - 2000 letters per hour), there was no means of readily checking individual sort (or missort) rates.