The third seat conversion has the third person sitting aft of the person in the passenger's seat...in other words, in the original baggage compartment. Hence the increase of the baggage area weight limit to 180 lbs, and the addition of seat belt tabs in the aft corners of the baggage compartment.
It helps if someone is actually going to sit back there (a uniquely bad idea in any but an emergency) to have a narrow passenger seat back in your rear seat, so that the aft passenger can rest their feet on either side of the "middle" passenger.
This modification originally came about as a result of the polar bear guides hunting off the coast of NW and western AK. They typically operated with two Super Cubs, one with the guide and client, the other with an assistant guide and lots of five gallon gas cans.....till some of these guys figured out how to increase fuel capacity.
Once they found a bear, the guide would land, and if that went well, he and the client would stalk the bear or vice versa, as the case may be. The assistant guide would land and if they killed the bear, they'd skin it and load the hide and skull in the assistant guide's plane, and launch both planes for AK.
On the other hand, if the guide broke his plane landing, the assistant guide would find a safer place to land, they'd put gas in that airplane from the cans, all three would board that plane, and they'd fly back to AK.
At some point, the FAA objected to the notion of three guys flying home in one two passenger aircraft.....hence the development of the third seat conversion.
In matter of fact, now that polar bear hunting with aircraft is illegal, the third seat conversion is most frequently used to stiffen the airframe in this area and to increase the legal load capacity of the main baggage compartment.
MTV