i should also clarify that the serial number has six digits. the ** represented the last two numbers. the book i read said it was made between 1958-1966. is that not correct?
If it is model marked MOD-20 then it is a rarity. They are hard to come by and a lot less common than post war guns. Can you show us some pics. A few of us here would love to see it, including the model marking.