The Alofs conversion is a contraption that can be bolted onto the side of a single shot break action shotgun to convert it into a 4+1 capacity repeating action. Patented by Herman Alofs in 1924 (https://patents.google.com/patent/US1507881), it was sold in the mid/late 1920s for $6. Surviving advertisements show it in the Iver Johnson catalog listed as 12ga, and in flyers directly from the Alofs Manufacturing Company offering it in 12, 16, 20, and .410 gauges. I am unaware of any examples not in 12ga, and I suspect the other sizes were advertised in hopes of finding buyers, but never actually produced except perhaps in prototype. These were only available for a few years before manufacture ceased.
Perhaps the most remarkable thing about the Alofs is that it actually works! As long as it is mounted properly (the mount is adjustable to get correct spacing and alignment), it actually does exactly what it advertises. One could buy a single shot gun and an Alofs unit for about half the cost of a pump action repeating shotgun. Of course, a true pump action was a much preferably option, but for those tight on money the Alofs was actually a mostly-viable option.