Here's the contents of the circular written out on the U.S. Flagship Hartford on the Mississippi River, April 17, 1862. I doubt the numbers were changed after passing New Orleans.

The Commanders of the gunboats will immediately have their smokestaks marked with the number assigned them. The figures will be painted white, six feet long, near the top of the smokestack, on the starboard and port sides, viz:

Sciota 1
Winona 2
Kineo 3
Wissahickon 4
Kennebec 5
Pinola 6
Itasca 7
Katahdin 8
Cayuga 9

H.H. Bell, Fleet Captain

ORN, Series 1-Volume 18, page 132