20.O23 [Houthulst Forest, East of Nachtegaal] December 26, 1917
Covers an area equivalent to portions of squares O22, 23, 28, and 29 on the corresponding WWI map sheet 20. Negative roll 9B, photo 203. Annotations on recto: north arrow pointing up. WWI Aerial Photograph Collection: Box 1, Envelope 59.