Time to add another scratchpad!
I finally found the spacer:
Civil War Era
Confederate troops crossing the fords of the Potomac in early September 1862 for the invasion of Maryland, which would culminate in the Battle of Antietam. (Print of a wood carving based on a drawing by Thomas Nast; first published in the September 27, 1862 edition of Harper's Weekly.)
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Union Defenses along the Potomac near Washington, DC
Top row: Chain Bridge (two views) and Pimmit Run Bridge; Bottom Row: Aqueduct Bridget {two views) and Georgetown Ferry
Top row: Chain Bridge (two views) and Pimmit Run Bridge; Bottom Row: Aqueduct Bridget {two views) and Georgetown Ferry
Union soldiers manning the Lower Battery at the north end of Chain Bridge in 1862.