I've used that ring on the FM before! The knurled ring on the FM was "intended" to make it easier to turn on the meter when using with the motor drive. I remember many FM owners running the MS-76 batteries dry because they forgot to turn the meter off when using the knurled ring. Probably why Nikon dropped it. The FM also has the distinction of using Gallium Arsenide Photocells. These cells have a better response to Visible light, do not have the IR response of SPD's. The Silicon cells required the IR blocking filter to be used for accurate metering, GaAS did not. Not many cameras used them.