Slate roof ridge detail

A slightly different ridge technique. Here there are two courses of slate laid flat and screwed to the ridge board.
The first layer / row is laid flat across the top field course with the opposite side ridge lapped over the top edge instead of meeting at the center. The second layer is offset from the joints of the first layer, and the top edges lapped opposite the first layer. The first layer has been nailed to the ridge board. The second layer has been drilled and then screwed to the ridge board with brass screws and washers.
This roof is close to a hundred years old and the ridges have not slid or opened.