I took photos of two of these nymphs yesterday: this is the second. The morphology this time really confirms, I think, the ID of Isonychia georgiae.
"Filamentous forecoxal gills each with single filament."
Yes. "Abdominal sterna with markings similar to any of Figs. 2.386-2.389." Figure 2.388 looks something like this. (.°.) In the middle of the sterna there are what look like parentheses marks. Above and between those marks there is a small pale circle, while in-between those marks at the bottom there are two smaller pale dots. Have a look at sterna 6-8 on our nymph.
Bingo! That pattern's as clear as can be, especially on sterna 6.
Finally, "Abdominal sterna 6-8 with markings similar to any of Figs. 2.386-2.388; abdominal terga yellowish brown to dark reddish brown (with or without lighter mid-longitudinal stripe)...... Isonychia georgiae." As we've seen, we have an exact match for pattern 2.388. I'd say the terga are dark reddish brown.
One other thing I might mention, that the gills are rosy/purplish in color is apparently also used in diagnostics. (See the comment by Roger Rohrbeck at http://bugguide.net/node/view/1387557.)