Swoody is exactly right. A fire jack is a set of grates to hold something up above a fire.
In the last picture from the link to the story, a fire jack is shown with wood ashes under it, and a pan sitting on the stand.
Attached is a advertisement lifted from the same May 1916 edition of "Recreation" that the story came from.
GREAT FIND!
and does confirm my suspicions
some of the guys over on BUSHCRAFTUSA have created a nice folding grate but it would take up more space in your kit, than this one
aint this fun?!
sw