This is an inside magazine tube for the pre 1975 Marlin Model 60, 99, and 99C rifles. These haven't been produced in many years. If your serial number starts with "26" or higher (or does NOT have a serial number), the outside tube measures 20 9/32" from the receiver to the end, and the "J" notch is 1/4" long - this is the tube! We had these made with a metal follower instead of plastic.
If your serial number starts with 11 through 25 then you need part number 507522 found here.