Your flag is not a guidon. There was a standardized system of command flags that indicated corps number, commander, division & brigade commanders.
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You will note that there is no pennant with a white border.
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It also does not resemble the Veteran Volunteer banners.
Without having the dimensions of the pennant at the head of this thread I can only speculate.
It could be a marker flag. Sergeants flew small flags from a fixed bayonet. It was used to indicate dressing to the center, right or left.
Often marker flags were makeshift. Early & late war is when depot made marker flags were most commonly issued. Because of their workaday purpose, marker flags are quite rare.
I would not be surprised if this pennant is a veteran's reunion item. It fits in with a GAR type of commemorative design.
I have never seen this exact pennant before, which is not entirely surprising. The Veteran reunion / GAR flag world is long on surprises.
Hope this helps.