There are three versions of Nikon FM: three-knurled-knob version, two-knurled-knob version, and single-knurled-knob version. The single-knurled-knob version, which is the last version, is the most reliable and easier to change focusing screen. The three-knurled-knob version and two-knurled-knob version have serial numbers starting with number 2; whereas the single-knurled-knob version has serial numbers starting with number 3.