The M60 machine gun had these variants:
T161: The M60's developmental designation before it was type-classified in the 1950s.
M60: The basic model, type-classified in 1957.
M60E1: An improved variant that did not enter production. The primary difference was the handle fixed to the barrel and the removal of the gas cylinder and bipod from the barrel assembly.
M60E2: Used in vehicles as a coaxial machine gun; electrically fired.
M60B: Used in helicopters in the 1960s and 1970s; unmounted.
M60C: Used in fixed mounts in aircraft in the 1960s and 1970s; electrically fired and hydraulically charged.
M60D: Replaced the M60B; a pintle-mounted variant used especially in armament subsystem for helicopters, but also some other roles.
M60E3: An updated, lightweight variant adopted in the 1980s.
M60E4 (Mk 43 Mod 0/1): An improved variant of the 1990s that looks similar to the M60E3, but has many improvements. It has subvariants of its own and is also used by the U.S. Navy (as the Mk 43 Mod 0/1). The Mk 43 Mod 1 is a specialized variant with additions such as extra rails for mounting accessories.
M60E6: A lightened and improved variant of the M60E4.
The M60E6 machine gun was adopted by the Danish Army, designated as the LMG M/60 with C79 LMG Optic. The shoulder thing that goes up (not a barrel shroud)
is shaped different than what many of us are used to seeing on M60s. Although I never even saw most of those variants, I did work on the original M60, and the M60D which had Ma Deuce-style spade grips on the rear. Those were used on helicopters, including 2 at the shoulder windows of the CH-47 Chinook (S**thook), as well as 1 on the rear ramp which was fired from a prone position, or sitting with your legs danging out the back. With a weight of 23 pounds, compared to the M16A1 at 6.37 pounds unloaded, without magazine and sling, no one I knew ever wanted to be issued the M60. It was like carrying 3+ rifles all at the same time. The M16A1, BTW, weighed an ounce
less that the M4 carbine with it's 5.5" shorter barrel and lightweight, collapsible stock, 6.43 lb empty, 7.75 lb with 30 rounds loaded magazine and sling.
The M60D shown is an old model with the stamped steel, "skeletonized" bipod base, which was soon replaced with the more robust casting.
https://en.wikipedia.org/wiki/M60_machine_gun