This should be thought of as a root-position seventh chord with a ninth added to it.
I know it isn't some kind of inversion, since the V9 inversions are indicated differently.
In fact, it's even easier than you thought. :) It's not an inversion, it's in root position. Basically, it's almost the same as just writing "7", which is also root position. But since F is in the bass, and G in the treble, the composer may have wanted to clarify that the G functions as the 9th of this F7 chord.
I think the composer could have gotten away with just writing "7" but decided to make it clear that it was an F7 chord with a 9th in the treble. Confusion may have evolved otherwise, as F and G can form a 4 2 G7 chord, (i.e., third inversion), and G7 is not too distantly related (V7/ii); while sight-reading this excerpt, there could be a gut-reaction to see it this way otherwise.