I have acquired numerous ETs over the last 45 years and I think only two had the front hold down installed. And yes, there is a gap. My guess is that the hold-down is just there to keep the batteries in place in the unlikely event of a rollover, so they were never meant to touch the batteries or the plastic battery cover. 

At least one of my tractors came with the rear battery hold-downs. A piece of about 1/2” angle iron that were on each end of the battery box, or ran a full length from front to back, depending on the tractor. For instance, I can see the holes for the hold-downs in the front and rear of the battery box of my I-5, E15, E16 and ETG150. However, one of the hold-downs I have is made to fit in a slot in one end of the box and a bolt in the other. https://www.myelec-traks.com/GEF-4374A.pdf