What is the German “1920” Double Date Stamp?

One will sometimes encounter German WW1 arms – mostly Lugers and Kar98 carbines – that have two date stamps, one of them being 1920. What is the significance of this? The crux of the matter is that the “1920” stamp is not a date, but rather a property mark. When the Treaty of Versailles was being enforced, it restricted the German military to just …