Hi mitzygolf
The number you have indicated on the casting is its part number not a machine number.
Atco never had registration numbers.
Your Atco 22 inch Standard started production in 1921 and finished in 1929, although Atco dealers were still selling the model as new to as late as 1932.
Through the production time line of the Atco Standard 22 inch, there were various modifications, mainly with the engine cc capacity and secondly to the carburettor used, most changes took place between 1921 and 1923, your machine dates to the latter production 1923 – 1929, as can be seen in your picture as it has the later Villiers carburettor.
If you wish to see a timeline in picture format then the best web site to view your machine is on http://www.hdtrust.org
Also you will be able to read the history of Atco, and also see pictures of the original prototype Atco Standard of 1920.
The only other way to identify your mower more accurately would be to have the original bill of sale, which some would say a tall order!
The adjoining picture is of the only known example of the Atco Standard 22 inch prototype 1920, take a close look at the frame
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