A lot of CS flags were taken from September through October 1864 by Sheridan's cavalry and some of this can get confusing indeed. Flags were taken at Third Winchester (September) and Cedar Creek (October) and in some smaller fights in between. The OR has a listing of these flags and where they were taken. Some 29 flags were taken between August 8 and October 31 by the Union cavalry. One was the flag of the 6th VA Cavalry taken on Sept. 24th by Lowell's Brigade and mentioned in his report. Included in the list is this flag taken at/near Tom's Brook on October 9th by the trooper of the 2nd US Cavalry which is WD 171.
The colonel (Crowninshield) of the 2nd Mass Cav in his OR report only mentions one flag being taken and does not ID that flag (see page 492, OR I, Vol. 43, Part 1). None of the regimental commanders of Lowell's Brigade filed AARs. Torbert's report only mentions the 5 flags taken in this time frame, that of Tom's Brook being one and does not name any of them. Merritt's report mentions no flags.
I will have to see if Google books has the unit history of the 2nd Mass Cavalry to see if the flag is named there but none of the Union OR reports name the flag taken by the trooper of the 2nd US Cavalry - although it is named in the chart of flags. How that got determined and how the capture was resolved between the 2nd US Cavalry and the 2nd Massachusetts Cavalry as there's nothing in the OR that covers it.