On some units, there is angle steel welded along the outside edge of the upper box steel frame. The bedroom wall sits on this ledge. Then there will be vertical screws thru the ledge and into the bottom edge of the wall. In other cases, the ledge is just a means of placing the wall and carriage bolts are installed horizontally thru the side wall and thru the box steel. If hung wall construction was used, then those bolt heads will be hidden under the fiberglass skin, but the nut should be visible on the backside of the box frame. I had such a unit that needed frame stiffening and with the front cap removed an extra carriage bolt was added thru the skin, but was then hidden once the front cap was re-installed. The carriage bolts were not crowned but an oversized flat head. It is also possible that horizontal carriage bolts were used but minus the angle steel ledge. And again they could be under the fiberglass skin.
