those are specially designed vent holes to prevent heat build up from the throttle by wire system. i think the coke bottle type nostalgia grips are very comfortable. i've had a few sets on my past harleys and i am using them on my streetglide. they just fit the hands great.
Those holes are there because all old grips had them. It has nothing to do with heat or anything else. The ends of the grips are rounded so I do not think you will find endcaps to fit them. Good luck.
thanks everyone. i probably err on the comment of paranoia about rain, spiders or bees. i will leave the well enough alone - unless i find some cool tassels.