The 601 was produced from 1958 and served in a couple of countries in Asia starting in 1959. It was the full auto but didn't have the ability to accept the rails that the Army decided they needed. It was stamped Colt Model 601. The biggest difference was the charging handle. The 601 used a diamond shaped charging handle while the 602 used the M-16 style t handle. The AF originally bought the 601 for use in Vietnam before the M-16 was named. Right around 1966, the army purchased the Model 602 while the AF purchased the Model 601 in 1962. What's funny is that the Model 750 (known today as the AR-15) was also introduced in 1963.
The Model 601 was a fully automatic weapon that was first put on the market in 1954 by
Armalite. The Model 750 was the same weapon except it could only fire a single shot per trigger pull. The Model 601 could do the single shot but had the 3rd selection it's selector for full auto. Interchangeable part between the Model 750 and the Model 601 in 1963 was more than 90%. Later, the Model 602 and the Model 750 were both upgraded for the addition of the rails. The Model 601 didn't have the facility for those rails
In 1968, the Model 601 (AR-15) was restamped with a simple restamping of M-16. Not M-16A1 which was the model 602. They also changed the Model 601 to accept the accessory rails of the Model 602.
View attachment 1063819
It was later restamped by adding the M-16 in the ID stamping. But it still carried the AR-15 stamp. Here is a bit of information. When the AF purchased the 601, it did not have the plunger assist. The AF purchased the Model 601 along with a cleaning kit stored in each butt stock. At the end of the day, it was common practice to clean your weapon. The Army left out the cleaning kit for their model 602 (M-16A1) The AF used new commercial ammo
M193 ammo. The Army used reloads from their own Powder which as originally meant for the Colt 1911A1. And we all know that the 1911 was one dirty shooting gun. Plus, the Army did not impress on their troops of the daily cleaning nor to clean it after you dumped it into a muddy patch. The Army M-16 had a jam problem and the the jamming assist was thought to be necessary.
This is not a complete history but it's enough for now.