In the second photo are men of the Sikh Regiment of the British Indian Army. All of them are sitting in the traditional cross legged prayer position. They’re probably reciting their final prayers as this picture was being taken. It’s very morbid if you think about it. The vast majority of Indian soldiers captured when Singapore fell were Sikhs. In the last picture you can see beards and long hair on the soldiers. The British found it in Japanese records when they retook Singapore.
A marker hangs over the heart of each prisoner and the stakes in front bear the of the rifle/gun. Each target position is marked with a number so that you can tell the soldier in position 1 to shoot at target 1 (and not at 2 etc). The position where the targets are located is generally called the Butts. On a civilized military shooting range, the targets can be raised up (when being shot at) or lowered down so that someone can mark the hits on the target, then patch the holes before raising the target back up.
This is a target practice, not a straightforward military execution by firing squad to me. A firing squad usually has a half dozen or more shooters per condemned, to guarantee a pretty instant death. These are assigned one per victim. You also don’t bayonet them at the end; if any are still alive, the officer should administer a coup de grace with a pistol.
With everything that happened in the Eastern Front, people tend to forget the horrific human atrocities committed by the Empire of Japan even before the beginning of World War 2. We only tend to remember the awful stuff the Nazi’s did but we forget about the unbelievable atrocities the Japanese Army conducted. The Nazis were methodical in their genocide. The Japanese (who killed twice as many Chinese as Nazis killed Jews) did it with pure barbarity. And while Nazi crimes were committed mostly by the SS and generally hidden from regular troops, Japanese war crimes were committed by regular infantrymen.