The most likely explanation is that it was used to signify that the mower was made or sold at the beginning of the season and hence followed the same design as the preceding year. Atco used the same numbering system across it complete range.
As with pre-1939 machines there was little need for a more complicated system as the company produced a relatively small range of standardiused designs.
Atco made relatively few different designs at any one time so simply knowing the size and year would be enough to identify the specification of the machine and which spares might be needed to repair it.
The numbering and dating of JAP engines is covered by this list of JAP engine dates and this alternative list of JAP engine dates.When mower production resumed around 1947 the company adopted a systematic approach to numbering its machines using a small detachable brass plate on the frame.This was typically located at the end of a frame cross-piece and held in place between the nut and frame.One consequence of this numbering convention is that it is possible to find very many mowers with seemingly the same serial number.
The Atco instruction booklets supplied with the mowers used the same coding system with one minor difference.
The simplest solution was to use a series of suffixes to the main numbering system.