Normally a hole would be there and the shelf would not. The shelf is there so that a semi auto trigger pack has something to engage. Clip and pinned trigger packs give the look of a full auto to a semi auto.
The reason why there isn't "just a hole there for a regular pin to go through? " is because ATF has deemed having the capability of affixing a swing down lower on this type of gun to be too easy to make into a machine gun and thus verboten.
On AR-15, the lower receiver is the gun for legal purposes. If you have a lower that is drilled for an auto sear, then it is a machine gun. On an HK, the upper receiver is the gun for legal purposes. You can have all the full-auto capable lowers you want. No problem. If the upper can accept a full-auto capable lower, then it is a machine gun. The shelf welded into the upper prevents it from accepting a full auto lower. The shelf actually occupies the space in the lower where the auto sear (and take-down pin) need to go. I guess you could in theory drill a hole through the shelf to accomodate a pin, but I'm not sure the shelf is really big enough for the hole. I don't have a gun in front of me. Either way, I wouldn't suggest taking innovative approaches to this stuff. It's not worth it.
Clipped is a functional modification. Pinned is cosmetic.
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