Just flashed on one possible approach to this puzzle.

Is there some way to find out just which tinclads where in proximity to the USS Marmora when the photo was taken?

How many other single-digit tinclads besides the USS Petrel (No. 5) might have been in proximity to the USS Marmora at the time the photo was taken...?????

just a thought......

y.v.o.s.,

Ken