Cnp field gun mud 01.jpg
Originally scanned from: GWS - The Great War: The Standard History of the All Europe Conflict (volume four) edited by H. W. Wilson and J. A. Hammerton (Amalgamated Press, London 1915)

According to http://www.firstworldwar.com/photos/sources.htm, “Each photo is believed to be copyright-expired except where indicated.” The site has been up for seven years and sources for the photos cited so it is reasonable this claim of copyright expired status has stood the test of time.

Note: other than the source of the image and the assertion of copyright free status, detail information such as credits for each photographer and image provider was not indicated for each image.
en
Cnp field gun mud 01, A British field gun., A British field gun.
commons
en
Cnp field gun mud 01, QF 18 pounder
commons
ja
Cnp野砲泥01、イギリスの野砲。
commons_translated
ja
クンプ野砲 ムード01、QF 18ポンド砲
commons_translated
four-pounder drilling mud bentonitic mudder asserted mud