In September 1869, there were 8 Major-Generals in the regular army acting down (until March 1869) to by cavalry field officers. They were:
Lt Col Elliott, 1st Cav - MG dated 13th March 1865 (nominated 17th June 1866, confirmed 23rd July 1866)
Lt Col Carleton, 4th Cav - ditto (originally nominated 11th December 1866 and confirmed 23rd February 1867, but recalled two days later, renominated 10th January 1868 and confirmation apparently backdated to the original confirmation)
Col Emory, 5th Cav - ditto (nominated 10th April, 1866 and confirmed 14th July 1866)
Col (from Lt Col, 6th Cav) Sturgis, 7th Cav - ditto (nominated 6th April 1868 and confirmed 17th July 1868)
Lt Col Custer, 7th Cav - ditto (nominated 30th June, 1866 and confirmed 25th July, 1866)
Lt Col Merritt, 9th Cav - ditto (nominated 30th June, 1866 and confirmed 25th July, 1866)
Lt Col Davidson, 10th Cav - ditto (nominated 17th July, 1866 and confirmed 23rd July, 1866)
Col Grierson, 10th Cav - MG dated 2nd March 1867 (nominated 30th March, 1867 and confirmed 5th April, 1867)
In rank order in the army (on 28th Feb 1869), they were:
Col Emory
Lt Col Elliot
Lt Col Custer
Lt Col Merritt
Lt Col Davidson
Lt Col Carleton
Col Grierson
Lt Col Sturgis
Custer had received a promotion to Major-General in the Regular Army in the summer of 1866, with the same date as Merritt, although he ranked above Merritt by dint of previous seniority (when the dates are identical, the previous seniority is preserved). There were, however, no slots in the Table of Organisation for him, and so he remained in regimental service as a Lt Col.
In the cavalry, Lt Col Elliot of the 1st Cavalry ranked Custer (which I missed) , with Custer being the 2nd most senior Lt Col by dint of Brevet. On 1st March, this changed and hence Sturgis jumped to being the senior and got the vacant colonecy.
Merritt survived long enough to make substantive MG by dint of seniority.
Interestingly, 4 days after the Act removing the 61st AoW was passed, Grant assigned several of those who'd lost their rank to
exercise it. I guess it took time to sink in.