Hi Charlie, there is an FS595 number for USFS Green, but I'd have to dig it up.
You can get "official" USFS Green spray paint from Kimball Midwest, this is what I've got at work. https://www2.kimballmidwest.com/All-Pro ... nts/80632/
You can get a very close match from Krylon Jade or Rustolium Seafoam. Honestly with the variations between manufacturer, and age of the vehicle these two are pretty much dead on and much easier to pick up locally than the touch up paint I linked to. Krylon Pistachio would do a good job if you wanted to do an older truck with some sun fading, being the right tone, just a lighter shade.
I've color matched with vehicles in our fleet and there is a noticeable variation between them. One engine has a mild 3 tone thing going between the cab, box and a professional repair all using "official" USFS Green paint.
What era is the IH you are looking at? There were changes in scheme in the late 50s and again in the mid 1970s. Changes in markings over the years as well.