I had another close look at the bullet holes in Photoshop, and I would guess, that they have used little stripes of any white cardboard, rounded it and put it into every hole. This could have been done, cause the picture was meant to be printed somewere (newspaper), and they knew, that in a low resolution print, the holes might have looked like rivets or anything but bulletholes.
I'd think this even more, as the hole in the (leather?) ring round the cockpit shows the same white rounding, although it's a different material.
I don't think, the plywood would splinter much, as it was certainly treated and painted to make it weatherproof. If a tracer bullet, rotating at high speed, would enter it, I should assume, it would leave a hole like a drill - rather neat and round and clean. Splinters, I'd only expect on the bullet-leaving side.
But well, it's all guesswork, still.