It is not easy to date Atco mowers made before 1939 with any precision. The company does not appear to have numbered its mowers or used any systematic dating process. There were relatively few Atco models and it seems that the company was able to run its extensive service operations by simply knowing the type of mower and the cutting width. From this could be determined the spare parts required and there was no need for any further complications.
All Atco motor mowers produced during this period had either Villiers two-stroke or JAP four-stroke engines. Both engine manufacturers numbered their engines and it is often possible to date a mower from these, bearing in mind that the original motor could have been replaced at a later date. The numbering and dating of JAP engines is covered by this list of JAP engine dates and this alternative list of JAP engine dates. Dating Villiers engines is slightly more difficult for two reasons. First, it seems that engines were supplied in batches. Second, many engines were supplied with a special number prefix which denoted their use on lawn mowers but offered few clues to their date. For example, many Atco Motor Mowers (The Atco Standard) from the 1920s have Villiers 147cc engines with "H" prefixes to the numbers. Later, and into the 1930s, smaller Villiers "Midget" engines have CY or similar prefixes that also seem to be mower specific.
For these reasons it is normally only possible to date Atco mowers to an approximate age within the known production period. Experienced collectors and enthusiasts can often give an assessment of a mower's age based on small variations in design.
Like many other manufacturers Atco ceased mower production during the Second World War to concentrate on other products. 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. All mowers were given a basic identification number with the format AABB/C where AA is the cutting width in inches and BB is the last two digits of the year of production. So, for example, 1247 would indicate a 12 inch mower made in 1947.
The final number after the / is not always present and there is some debate among collectors over its significance. When it is present it is typically one higher than the second "B", for example 1455/6 or 1754/5. 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. Occurrences appear to support this theory.
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. However, it should be evident that the basic numbering system did not allow differentiation between different models (or completely different types of mower) with the same cutting width. For example, the company made 12 inch conventional, rotary and battery mowers at the same time and needed to make a distinction between each. The simplest solution was to use a series of suffixes to the main numbering system.
This numbering system appears to have fallen out of use by the late 1960s and, as far as is known, no mowers produced after 1970 were supplied with these designations.
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. When rotary mowers were introduced by the company in the late 1950s they were designated with a similar number followed by an "R" - for rotary - to avoid confusion with conventional machines.
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. For mowers with a different cutting width but otherwise identical specification the manual would have carried what might be called a compound number, for example 14172055 to designate the booklet for the 14inch and 20 inch models from 1955.
It seems that Atco began to emply a longer serial numbering system from the late 1960s onwards. We do not have any details of this at present.