The date that I find--from Dyer's Compendium to NPS sources to a few miscellaneous sources--is 13 December 1864, which seems to be the date for reorganizing several black regiments.

Part of the trouble--as you may know--is that there were two 79th USCTs. They are often separated by the words (old) and (new) after the unit designation.

The 79th USCT (old) was created on 4 April 1864 for a regiment in the deep south. My understanding is that it never reached full strength, and on 28 July 1864 the designation was taken away and the existing companies consolidated with other regiments.

The 79th USCT (new) is the First Kansas Colored Infantry.

By the way, this old-new problem also exists for the 83rd USCT, the new regiment in that case was the Second Kansas Colored Infantry.

I can't offer an explanation why Ross would used the 79th USCT designation, unless the report was actually written later and he forgot to use the earlier designation.

And a piece of advice--at the Military History Institute at Carlisle, PA, they have photos of the white officers and a black drummer boy from the 79th. I was so pleased to hear of these photos, but then I got the names of the officers, and they did not match up with officers in the First Kansas Colored. Yep--their photos are of the old regiment, not the Kansas regiment. I've seen one article--I think it was in North & South a few years ago--that made the error of thinking they were using photos from the Kansas regiment.