Post-1975: This is an inside magazine tube for the Marlin model 60, 99C, & 990 rifles. It replaces Marlin part number 507522. It is 20 7/8" from the tip of the knurling to the end of the tube and 22 1/8" overall. It fits serial numbers that begin with 11 and go through 25. Pre-1975: This is an inside magazine tube for the pre 1975 Marlin Model 60 rifles. These haven't been produced in 38 years. If your serial number starts with "26" or higher, 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! Marlin Magazine Tube Mfg: The Gun Garage SPECS: Made for post and pre 1975, model 60.