The M203 was in service since at least the early 70s. I mentioned it earlier but Larry served since the early 80s. ......20 years in spec ops, 15 of those in Delta (Panama, Desert Storm, Somalia, Bosnia, etc.). He knows what he's talking about. He mentions early on in the video that the barrel shown existed prior to the M4 but was later known as the M4 barrel. Just remember that the only reason I even linked that video was a supplement to the discussion about the modifications some of these soldiers were making especially in spec ops where they had way more leeway on such things.
CoD Cold War covers the 80s with flashbacks to Vietnam and so on but it seems more focused on that period in the 80s in the maps, weapons, accs, etc.
Here's another example of a CAR15 with an M203 attached:. A different muzzle on this one compared to Larry's example:
SORRY.... Don't know what's going on with not being able to copy and paste images sometimes after the update.