I've always wondered about that. Looking at color photos taken from the period, they looked like they were black but was always told by the "experts" that "no your color blind, that truck is just really dark OD."
I read or heard that some officers complained about the black trucks. The answer was that the "book" specified that they be painted OD which is a blend of yellow and black. The "book" supposedly didn't give an exact formula, so the guntrucks, while appearing black, actually had a little bit of yellow in them, making the color a very dark green and thereby adhered to regulations.
Hey Smoke, it's been a long time since I got on here but I finally found a pic of the M-60 mounts. Looks like they were just the top of an infantry tri-pod welded to the top of the bed. Hope your truck is coming along OK.
For what it's worth, I wanted mine really dark too, but I wanted a hint of OD also, so I mixed it 3 parts 24087 and one part gunship black. Turned out cool, the color looks "bad". Check it out on my web page. Scroll down on left side and click on military vehicles. www.hoosierhotshot.com