"In other words that rifle had to have been made either in 1918 or 1919, maybe a little earlier, but it looks like from what I read it was only open for 3-4 years during WW1"
It was probably made before WWI. During that war, I doubt they were making .22's when every resource was needed to make military rifles. And after the war, production stopped, even though the factory was not formally closed until 1921. So, I would think pre-1914 would be most likely.
As to a pre-WWI German bolt action rifle being chambered for a proprietary American semi-auto cartridge, I can only think it seems unlikely.
Jim