The British ones are what were known as trade pattern bicycles. Various MK's and built by various firms BSA-Phillips-Royal Enfield-Mercury etc. lots were also supplied to the army post war and I think I'm right in saying that the wartime bikes had 28" wheels whereas post war had 26" wheels. Most wartime bikes would have had a stirrup front brake and coaster rear brake (pedal back) so only one handlebar lever. The rear carriers are usually dated.
Thanks for your interest, they are clearly American bicycles with 2" tyres, double cross bar and pull back handle bars. All the other equipment appears to be of WW1 vintage, uniforms and rifles, only the bored officer is up to date. Americans with bicycles pretending to board a dummy DC3, how they must have enjoyed that!
Richard