Footwear

Very probably. When this movie was made, one could go to a surplus store in anytown, USA and equip an entire infantry platoon with WWII surplus clothing, including shoes.

Having served in the infantry and collecting & selling collectible militaria for several decades, I can tell you that while durable, many military articles of clothing and equipment are NOT comfortable, especially when you begin to sweat.

My guess is-the stars, particularly pretty boy Tony Curtiss balked at wearing clunky and sweaty leather boots and thus demanded that the director get him something more comfortable such as black-painted sneakers.

Just a SWAG on my part, but a pretty damned educated one!

I've always thought they were Japanese boots, so they wouldn't stand out on a Japanese held island.

If you can read this....Thank a Teacher. Since it's in English.... Thank a Soldier.