They are as confusing as the D.M. Lefever doubles. A while back we were discussing them on one of the sites, and every picture we could come up with of one they looked different -- three pins/screws thru the side, two pins/screws thru the side and no pins/screws thru the side. They did have a solid rib. The first offering of a vent rib I've found was about the second or third year the Remington Autoloading Gun was offered.