Hole in Piston (OR NOT)

[jeep_n_john]

Well, Started disassembly of my spare engine. Mainly after the cylinder head as I have a broken bolt on the water pump thermostat housing and having difficulty getting it out. Anyway, As I took the cylinder head off, the cylinders in my spare engine did not look to bad. However, I noticed what appeared to be a hole in each of the pistons. Looked odd, as it was a very neatly hole like it was drilled into the piston exactly in the center. (see Photo). The hole was in each piston. Later, I looked at my 2nd spare engine which already had the cylinder head off and there were holes in those pistons. In reading a thread from someone else who was working on there 47 cj2a, his pistons had a hole in them also. So maybe it is not actually a hole, LOL. Can anyone explain what this is, Never really seen this before (I admit that I work mostly on Motorcycles, LOL)20221028_181720 resize.jpg