Marlin 99 wo/ serial number

I just took a old Marlin Model 99 semi-auto, tube fed 22lr in on trade. When I went to enter it in my book, I could not find a serial number. The gun looks new enough that it should have a number. Does anyone know when Marlin started putting serial numbers on their guns. I think this one was made between 1959-1960. Any help would be appreciated.