Machined hole in top of piston. Why???

Yabut the sound;
Ba-bam------shugga-shugga------- Ba-bam-------shugga-shugga---------Ba-bam
ooooooooooooo
Lessee; 90hp, 110hp, 120,130,140, and
a 5hp transmission built in 1947.
I built a lotta 4 speeds back in the day.
Don't even talk about clutches.
But in fairness, those engines do make monster torque, compared to the lil rice burners............ and that is very attractive; well that and the 7000pound flywheel..... that is held on with one lil nut.