When I was a kid, I bought a set of 6 colors: black, light gray, intermediate blue, olive drab, neutral gray, non-specular sea blue.
My guess is these were a set meant for USAAF/USN aircraft, and thus the OD was HU2.
From the few items I have left upon which I have applied this color (before the tinlet dried out - I guess my tiny fingers at that time could not close the lid properly) my impression is that this is a stronger and darker color than Humbrol 66. Also, I have wondered about the existence of HM3, supposedly an OD for US vehicles and tanks. It would seem that these should not be considered identical, after all why produce two different numbers with identical paint? Is there anyone out there who still has a workable tinlet of HU2 and/or HM3? How do they actually compare to a recent Humbrol 66, and to each other?
If someone also has an Airfix M21, I'd be interested to know about that as well.
-Lasse (obsessed with the mysteries of Olive Drab for more than half of my life.)