No the holes are not an original CCKW feature and I first thought they were foot holes to aid personnel getting into the rear but if you notice where they are placed they would allow the tail lights to show through to the vehicle behind when the tailgate was in the lowered position. Handy if you had a long load and had to travel with the tailgate down.
This is the first time I have really seen this type of cutout. Mind you it could well be I just wrote them off as foot steps rather then the showing of the lights with the tailgate down.
Cheers
Cliff