Not sure whether the OP has given up on this thread or not.

If so, FYI, I'd insert a seatpost to the correct position for my saddle height, make sure the post is rotated correctly so that the saddle is straight, and then drill the seatpost at the point in the center of the lollipop opening at the bottom of the back of the lug.

Then I'd thread the drilled hole in the seatpost for a 5-mm or (preferably) 6-mm bolt.

I'd use a washer under the bolt head and torque the bolt sufficiently that the stress at that point is distributed both vertically (below the lollipop hole) and horizontally (through the lug wall).