I am an avid collector of these NSW cut-down date-stamps. "jubilee's" introductory statement (2017) in another forum post needs some clarification for anyone interested in these types of strikes. "jubilee" stated that these cut-downs were used during the 1930s - 1950s. The period of usage was actually from the 1910s - 1960s.
...and cut-downs were not restricted to only single letters. Two letters were used and eventually numerous three letter examples. Several cut-downs are also found with the small, nylon type date-wheels substituted for the larger steel type.
We know that two date-stamps, labelled "POST OFFICE NO. 1" and "....NO. 2" were the precursors to the cut-downs. These two were first used in 1913/4 ergo the cut-downs would have followed shortly thereafter.
As shown in the spreadsheet screen grabs below, the single letter cut-downs were actually first used as early as 1916 - as discovered in my research. Usage might go back as early as 1915.