The holes are just an easy way to recognise the "best sounding tube" of the many versions in the 6H8C family, just like the round plates in the Tung-Sol round plate 6SN7(VT231) visually identify one of the best if not the best sounding NOS US made tube. I believe that all other US 6SN7 tubes have square or rectangular plates, even Tung-Sol made square plate tubes.
If you are asking what the round holes actually do for the tube, I can guess but do not really know.
