I suspect it's mostly field manuals and exported references. With older systems, it's easier to get hands on those as they've been exported to so many places a

I suspect it's mostly field manuals and exported references. With older systems, it's easier to get hands on those as they've been exported to so many places around the world. I know DCS for example has had trouble getting their hands on manuals for current Russian equipment (there's explicit laws on it that's hampered a lot of their development there), but for older systems, there's a lot you can find outside the country of origin.