From the Uzitalk library:
I don't understand how the bigger SA hole keeps you from attaching an SMG lower. My (FA) Uzi has a 9mm hole, but I can attach an SMG lower with its 8mm pin and it feels like it's in there solidly.Large Trigger Group Lug/Pin:
All the semi automatic UZI's have a 9mm hole in the trigger group lug that is welded on the bottom of the receiver, and a 9mm pin that goes through the trigger group and the lug. This is to prevent installing an SMG lower (which have 8mm holes) on a semi receiver.