In the US military, when was the nomenclature change from "Model of" to "M"
To the best of my knowledge, there never was a "model of" designation on any product issued to the military past the Civil War.
For example, the M-1 Garand carries its M designation because it was developed for and first used in the military. All branches of the service used this reliable (but very heavy) rifle. It is still in use where weight is not the primary concern. The M1-A is a smaller and lighter version first issued in the early days of the Vietnam War.
In some cases, specific products have been issued to specfiic branches of the military and carry the branch designation.
For example, the Hughes-Rayhteon AIM-9 Sidewinder Missile was designed "A" and first issued to the Air Force, hence the designation. The Sidewinder (and later AIM-9-C Chapparal) missile systems are still in use today (and brutally effective let me assure you).
Rocmike aka Yechiel nice life you have led Julius Watler.