David,
A property stamp consists of all 4-digits "1920". In the example given, the stamping is "1918/20", i.e. the date of initial parts manufacture, "1918", is followed by a slash (/) and then the digits "20", the date of completion. The "20" is usually a smaller font than the initial parts date. Your example, 1917/1920, indicates a 1917 date of manufacture with the subsequent 1920 government property stamp applied. To further confuse things, there are Lugers actually manufactured in 1920 that have a "1920" chamber date (there are even some of these that were subsequently property marked, i.e. 1920/1920!)
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